Watch out for other interesting physical changes as body is willing to speed back up to use those extra calories.

Workouts getting better - if you have a way of measuring intensity/speed/weight to compare.
Hair/nails growing faster. I always notice on toenails first since they are slowest anyway, but also speed of healing of cuts/scrapes. Keeping warm easier, ect.

Bumping.

Muscle. Kiki (from Eat More Weigh Less) just did a persicope about this very issues. When you diet down, you lose muscle, some water, fat, etc...lots of stuff. And then your metabolism slows down (becomes efficient...remember, your body wants homeostasis). Building muscle and lifting weights help to preserve metabolism. As does taking breaks.

FYI..I am cutting at 1950-2k.

Also can depend on how the majority of your daily calories are burned.

A reasonable deficit with resistance training and enough protein can prevent muscle mass loss, but you'll still lose LBM which effects metabolism. But need about 5-10 lbs gone for that to add up enough.

But if the vast majority of your calories beyond BMR are from increased daily activity - that is effected the most from moving around less weight all day. But with Fitbit synced you are informed of that fact.

Exercise likely goes up in intensity enough to compensate for moving less mass around, unless someone gets on treadmill and always sets the same speed. Which also means they have stopped improving their cardio system.

But even with that exercise aspect - even if big majority of daily burn is not from daily activity - your TDEE still drops as weight drops, even so slightly.

You can run normal TDEE calc with lower weights and see it doesn't change much.
That assuming you maintain same ratio of fat/muscle that BMR calc's are assuming.

It could be you are keeping the muscle, so BMR doesn't drop actually, only TDEE from moving less mass.

But now the deficit has actually increased if you think about it as weight has dropped.
So if body was on verge of not liking the deficit level before and on verge of adapting slower, that would likely cause it. But, usually that means the deficit is still there though, just as not big as might be suggested by just the numbers.

So say that 2330 was your actual TDEE on non-exercise day. And BMR really was 1864 (this suggests you are taking more deficit than your TDEE allows room for).
You eat 1830 and really lose 1 lb of fat a week for 5 weeks. Obviously during that time the rate of loss would slow slightly.

So you lose 5 lbs, but you maintain muscle mass, so BMR stays at 1864.
And TDEE now is say 2236.
So eating level would be 1736 to keep 500 deficit.

BUT - what MFP and even Fitbit (since it uses BMR too in calorie calcs) do is say with 5 lbs loss, BMR drops to say 1750, and sedentary TDEE at 2188.
They assign eating goal of 1688.

You would have a real deficit of 548. Again, not much difference for 5 lb gone with made up numbers, but you see what happens when reality is better that calculated.
And if 500 was almost or already stressful, that effect could cause stress to be bigger - depending on how much muscle mass you have compared to average already.

My Katch BMR (based on Bodpod BF%) is almost 200 more than Mifflin calculated. My last VO2max test where you sit still at start and can calculate RMR from the stats, bore out the accuracy of that figure.

So if I go by unadjusted Fitbit and MFP - I've got a much bigger deficit than reasonable, and prior to Fitbit giving a clue about daily burn, I was accidentally with a big deficit.

So to your X-Y to lose weight.

Both should become smaller as you actually do lose weight - and it could very well be depending on several factors - that the eating goal stays exactly the same. That is possible as deficit becomes less to match the decrease in TDEE.

Taking breaks is so much needed for the aspect of body adapting - even with reasonable deficit - you may have enough other stresses in life the additional diet stress is too much.

Either every 6 weeks take a week off.
Or with doing a dynamic eating goal daily with Fitbit with constant deficit amount - take 2 rest days and eat at no deficit. 1 day usually not enough if body really wants to adapt.

That's why the 5:2 diet showed such success in studies - 5 days no deficit, and eating to satisfaction. More than compensated for the 2 rest days with 75% deficit.