You lost 21 lbs in 45 days.
Lets say 4 lbs worth was the normal water weight, assuming you had a high sodium diet prior and lowered it a lot.

17 lbs in 45 days. 6.4 weeks.

Always do the math for big weight gains or losses, to see if it could have been all fat, or had to be other.

17 x 3500 = 59500 / 45 days = 1322 deficit from daily burn if really all fat.

Depending on true average eating level, below 1600 to above 2000 is rather huge range, though it sounds like you are more on the lower end.
1700 + 1300 = 3000 burn as you mention.

That's a 43% deficit. Does that even sound reasonable, eating 45% less than what your body would like to maintain at?

And actually, I'm going to bet it wasn't all fat that was lost, when you go that fast, some is going to be muscle mass, even with lifting. That can help retain muscle mass with a reasonable deficit and enough protein, it won't prevent it though.

And when in a state that your body feels the need to lose muscle mass, it's probably also adapting to that extreme deficit by slowing down and becoming more metabolically efficient.

So besides daily burn lowering because of less weight being moved around, metabolism has also lowered because of less muscle, and lowered more to adapt.

You are now likely seeing the results of that, you no longer have the deficit you thought you had.
Let's see, if you take off the 20% suppression studies have shown to occur, assuming your 3000 is correct for prior potential TDEE.

3000 - 20% = 2400 merely for suppression.

Eating level 1700, that should be still a 700 cal deficit, or almost 1.5 lbs weekly.

Take off lost muscle and less mass being moved, that could easily be lowered to 250-500 cal deficit.

Yes, eating at 43% deficit from potential daily burn is doing something wrong, and it's already bitten you.

The problem now is 2 directions.

You can keep eating less, and that max suppression of 20% remains, so you eventually will start losing again. But how low must you now eat combined with how much exercise to cause that loss?
Get sick or injured or vacation and little to no exercise - now how low must you eat?

And play out the end game since still some weight to lose, if already at 1600 calories eaten, how much will you have to eat to keep losing?
How bad will maintenance be?
Will you be able to adhere to eating levels much lower then they need to be?
To play out the end game, your TDEE will lower about 75 calories per 10 lbs lost. If it is indeed already at 2400 for your current level of activity, what does that mean TDEE will be at goal weight, along with how much exercise?

I'm hoping all that scares you, because it should. That is exactly why majority of dieters fail, maintenance sucks for such a low eating level.

The other direction is heal up the body so it's healthy and can burn maximum amount. At this point losing muscle mass you already lost some of that extra burn, but oh well, lesson learned - probably don't want to cause any more loss, huh.

Also remember that eating too little messes with your hormones regarding hunger.
You feeling full, and your body being fully fed for your level of activity - are 2 very different things.
You ever feel like you are getting vitamin or mineral deficient? Or rather you'd have symptoms show up down the road eventually that cause you to get tested and point out what is wrong, perhaps with some serious health issues as a result of it.
Don't become calorie deficient, there are even worse potential side effects besides the ones you've experienced.
You probably lucked out on some because you indeed have a bit to lose - but that doesn't mean the body is happy about it, and won't do things to correct what it thinks is wrong.

http://www.youtube.com/watch?v=2i_cmltmQ6A

So which direction are you interested in, really interested in?

Spreadsheet on my profile page to get best estimates of BMR and TDEE based on bodyfat measurements (or your own tested level), and activity calculator that is specific to you and time and exercise.
Stay on Simple Setup and Progress tab, and see what TDEE looks like it could be for your BF%.

And then you likely need to start eating about 100 extra daily for a week or two at a time to work your way back up to that value to get a healthy body back, that can then sustain a reasonable weight loss amount. And yes, reasonable for you will be more than others, but you can still go overboard.

thanks everyone... here's my next question. I only have a fitbit plus and myfitnesspal to "guesstimate" my calories burned during a workout session. We all know these are not accurate so is it better to set my calories higher (at like 2500) and choose my activity level at moderate and HOPE that I really burn 3200 per day or to set my calories at my BMR level and then eat back my exercise calories as I earn them (again, not accurate). This is where I'm struggling. I'm sure there is some trial and error here as well.

There is no Fitbit Plus, but whatever device, it's a better estimate than you guessing from 5 levels of TDEE chart or MFP non-exercise activity levels. It's giving you daily infinite TDEE levels.

Yes trial and error, erring on the high side, will get you there - but are you really willing to wait?

From studies, the Fitbit's can accurately log 85% of average daily burn, the other 15% depends on how much you exercise, eat, ect.

Because it actually underestimates.
ALL non-moving activity is given the same burn, BMR based on gender, age, weight, height, what you'd burn sleeping deeply.
But actually when awake you burn more - RMR.
When standing you burn more.
When fidgeting you burn more.
When processing/digesting food you burn more (about 10% of what you eat).

So it actually underestimates daily life, and lots of exercise it does too, because while step based, it has no idea of incline for walking or jogging, which can be a big difference, and of course really underestimated on lifting, biking, elliptical, rowing, ect.
All non-step based must be logged manually to increase accuracy of estimated daily burn.
So confirm you do that, or you'll be creating still a bigger deficit than your body really wants.

Lifting should be logged on Fitbit as Weight training, if it's heavy for you with sets and rests, then that is power lifting, if you are doing it easy - make it harder and use that one! ;-)
MFP calls it Strength Training.
Both have circuit training if that's what you are doing. Normally that would be best for fat loss as it increases your TDEE more, but yours is high already, so perhaps you don't need that.

And like Jen recommended, you still are commenting on mentality of lower is better (setting to BMR), hence my thought you aren't going to estimate and wait on high side to show results to adjust from.

If you like the eat back method (which is frankly closer to actual TDEE deficit method, but harder to plan since each day is different calories), then set it up correctly in MFP, and sync with the Fitbit for both positive and negative adjustments.

Select correct non-exercise daily activity level.
Sedentary is 45 hr desk job/commute weekly, with no kids or pets playing with, no lawn care or long shopping sessions.
Lightly Active is desk job with kids/pets and more house cleaning/cooking that comes along with that, and longer shopping (not with pets perhaps).

Select 1 lb weekly weight loss, as that is about 15% of what appears to be TDEE right now, though I'll bet it's actually higher than you think.

Log your foods accurately - weigh foods, measure only liquids.
Log your non-step based exercise correctly. Let Fitbit handle the step based walking and jogging. So you'll have to track when you start lifting, not the cardio warmup, or walking and stretching, but from first lift to last lift (do count reasonable rests, not talking & waiting 5 min for equipment). Was spin bike really 45 min, or last 5-10 min was easy spin and stretching? Was Zumba class really 60 min, or 15-20 was warmup/cooldown slower (1/2 time) and stretching, so really 40-45 min?

Since you have Fitbit, the main reason I'd still suggest using the spreadsheet in Excel or Google sheets if you have gmail account, is to compare Katch BMR based on BF%, to what BMR Fitbit is using - could be close, could be way off, in which case correction would be useful. And on Progress tab is a TDEE calculator based on actual results (need accurate food eating logs), to see how close Fitbit was.

If you prefer the TDEE deficit method, same amount daily, then you'll still need corrections on Fitbit's site to see what average TDEE is, then take 15% deficit and manually set your goal.

With either method, you need to slowly work your way back up to TDEE for at least couple weeks. You'll know if body is responding well as you move up if your weight loss increases a tad more before you reach TDEE.