Still kind of lost on getting to a "good" level of cardio

Um ok wait. You're eating 2,030 calories a day, and burning 2,100 calories a day doing cardio.,...meaning you are netting -70 calories every day???

Sounds like gymorexic, gymnasia, or whatever they call it. And the "feel like my body's never been normal" kinda raises a flag to me. Get some pro help, dude.

You are probably not tracking your eating accurately, or not tracking your cardio accurately. I have found that for me, MFP overestimates my burn by about 30-40 percent, so I created my own exercises with more-accurate burns. Once you track your food as accurately as possible, the next thing to do would be to just decrease what you eat gradually until you start losing again.

to much cardio my friend! bring your cardio down to 30-45 min a day after your weight training of course. You want enough energy to get you thru your weight training that way you wont be bummed out if you do cardio before hand. also check your sodium intake if its to high that's where your extra pounds are coming from. Water retention from the sodium. Try a cyclical ketogenic diet. you low carb highfats and protein for 6 days then have a high carb day on the 7th. This really gets the fat off quick.

I'm sorry dear, I don't think you're burring 2100 calories a day doing 3 hours of cardio

I easily burn 700+ calories doing and hour of plyometerics, kenpo x(maybe closer to 600), or an hour of insanity.

You don't have to do that much cardio, especially for your weight. It is very possible to overdo cardio, and you're definitely on the verge. I would suggest doing cardio, but make sure you're not getting tired, or run-down. When you do, it's time to stop. Also, you're body can go into "starvation mode" if you're not eating enough calories, because it thinks it isn't getting the proper nutrtion. Take it easy, the scale will follow, take it a day at a time, and just make sure you're burning more than you're taking in. Now, that doesn't mean exercise all the food off, because you burn calories every second from breathing, etc.

Unless you are doing a submersion body fat analysis or something at specialized clinics, fat% is a close guess with calipers and flat out wrong with the electronics. Those things should be "for entertainment purposes only".

Given how lean you say you are, you should just accept yourself in the mirror and your clothes. It's a slippery slope to an eating disorder when you are pushing certain boundaries.

You are under eating and over exercising. You cannot continue like this forever. Not a lifestyle change.

I'm sorry dear, I don't think you're burring 2100 calories a day doing 3 hours of cardio

I easily burn 700+ calories doing and hour of plyometerics, kenpo x(maybe closer to 600), or an hour of insanity.

Yeah, but do you do 3 hours in a row?

I mean, maybe you do! I would be totally jealous of that burn if so!

I agree. Even if I ran for 3 hours straight, I don't think I would hit 2100.. And that's running at 7.5-8.0 miles per hour

"Most weight loss occurs because of decreased caloric intake.
However, evidence shows the only way to maintain weight loss is to be engaged in regular physical activity.
To maintain your weight: work your way up to 150 minutes of moderate-intensity aerobic activity, 75 minutes of vigorous-intensity aerobic activity, or an equivalent mix of the two each week."
(The page explains moderate & vigorous.)
http://www.cdc.gov/healthyweight/physical_activity/index.html

I've still been doing about 3 hours a day of cardio. Comes out to around 2,100 calories burned. Also do weight training every day.

Don't do weights every day, or rather don't exercise the same muscle groups every day.
Allow at least 1 day in between for your muscles to heal. 2 would be better.

I've kept my eating incredibly strict, and I consume 2,030 calories a day.
I've held this rate by eating the same foods for the past... four months or so? No change to diet.

Sounds incredibly boring & restrictive.

8 months ago I was 9.9% body fat, at 180 pounds. I got it tested again around four months ago, and I was 8.7% body fat, at 172~ pounds.
I still hover around 172 pounds. Sometimes more, sometimes less. The fluctuations scare me.

So you're doing very restricted eating, massive over-exercising, you're at an extremely low body fat, and you're scared of weight fluctuations.
Sounds like an eating disorder.
Have you talked with your doctor or counsellor?

Is it possible to gain because of undereating? Or overdoing cardio?

No.
You gain weight because you're eating more calories than you burn (which does not sound like your situation, at all),
or because of water retention (which is most likely with this sudden small gain - check if you've had something with extra sodium lately).

I'm just scared to do less, but at the same time everything tells me that I'm already way overdoing things.

Counselling. You know you're harming yourself, yet you're scared not to.

I guess I'm a bit freaked out at a sudden gain over two weeks.

6 lb in 2 weeks is almost certain to be water, not muscle or fat.

MFP overestimates my burn by about 30-40 percent

Yes.
Yesterday MFP says I burned 667 cal on the elliptical (when it only knows my body weight & the time, not the speed),
yet the machine (which knows weight, time, and speed) says I burned 471.

Way too much cardio. How on earth do you find the time to do all that? I'm lucky if I can squeeze in 30-45 minutes a day.

If it takes 3 hours of cardio a day to get fit I'm throwing in the towel right now... :noway:
I don't have the time, energy, motovation, or the want for that...

It doesn't.
The CDC says 30 min per day to maintain weight, maybe twice that to lose.

"To maintain your weight: work your way up to 150 minutes of moderate-intensity aerobic activity, 75 minutes of vigorous-intensity aerobic activity, or an equivalent mix of the two each week."
(The page explains moderate & vigorous.)
http://www.cdc.gov/healthyweight/physical_activity/index.html

HOLY WOW - like seriously where do you FIND 3 hrs to do cardio?!

It kind of sounds to me as if you may have hit a plateau of sorts and despite what logic may lead you to think - 'hey I need to do MORE cardio' what you may want to do is to rev up the intensity in a MUCH MUCH shorter duration.

When I hit a plateau I kept extending how long I was doing cardio. At one point, being a full time working mother of 2 I simply couldn't afford more time in the gym. My friend who had just received her masters in nutrition and is a personal trainer suggested that I reduce the amount of time but rev up the intensity which I did; I had nothing to lose at that point because I was SO frustrated with the lack of progress.

And wouldn't you know it ... in a matter of a week I broke through the plateau that I was stuck at for over 2 months!

Also as many of the other posters have mentioned, if you over exercise your body will revolt because it thinks it's going into starvation mode.

Additionally, because you are exercising SO much, what sort of calories are you consuming post workout? We can't out gym poor eating choices - no matter how frequent or the extent the duration we invest at the gym.

This lifestyle change should be about finding balance in how much you consume and how much you exercise. From my experience an extreme in either or never seems to have the desired results we are all ultimately wanting.

Best of luck to you.