Starvation mode, adaptive thermogenesis, metabolic efficiency - isn't about the eating level - it's totally about the amount of deficit from what the body could maintain on.
That's the threat.

The effect has been seen, regularly enough it's expected, in morbidly obese patients put on a diet to lose enough weight to have surgery. Their normal TDEE may be 4000-5000, and when they feed them 2000-2500 (50% deficit), which sounds like enough, they expected after a few weeks that the TDEE will lower not only from less weight, but by good 20-25% because of metabolic efficiency. So weight loss will slow even more then weight would indicate. That's why they hope to add exercise at some point, to offset that and increase TDEE back up by being more active.

And no, it's not possible to gain from eating too little - that's a myth.
What happens is someone has suppressed their daily burn so much they are eating at maintenance.
Then on a regular basis they binge from eating so little. That doesn't raise their metabolism, it stores the extra as fat, and weight goes up.

Starvation mode is merely your body suppressing it's burn. That's what you did, below what it could have been.
Now you are trying to unstress the body so it'll increase that back up.

As to what can add weight that fast.
Water weight as you said, increased carbs in the muscles finally, increased blood volume, ect. True on carbs, unless doing endurance training, and even then could only be maybe 2 lbs max. Blood volume another lb.
Not increased muscle, not undereating. Body isn't going to expend precious energy it's not getting enough of to build more muscle that will require even more energy to maintain when it's not getting enough now.

Glad you did the math - when you spell it out, it's very ridiculous to think of maintaining on 1500 if the weight gain really was fat.

You do you have any health issues the body is also dealing with?
Because stress, from under-eating, from food allergies/sensitivities, from lack of sleep, from life, ect, elevate cortisol that can help retain upwards of 20 lbs of water.
And if illness or disease then there is even more stress.

So you didn't increase slowly, but jumped straight up to 2000?
And you were eating how much with maintaining weight?

So you actually do have a surplus then until such time as your metabolism speeds back up. That difference is real fat gain actually.

Today I had my body composition measured by a professional in the hospital and it turns out that the last 10 lbs I gained were all fat!
How the hell is that possible? In those three months that I gained those pounds, I worked out about 5 times a week. I thought that at least some part of the gain would be muscle, but it turns out it is not. Does this mean my maintenance is really 1500 calories only? With the exercise I'm doing that sounds really ridiculous.

Yes, it is ridiculous, so let reason enter in to the initial reaction. No need for more stress.

Body comp measured how exactly?

And measured the same way at first that you are comparing too?

Every method has range of accuracy, and if on one side of range first time and other side this time, totally false gain in BF%, and merely in the overlap of inaccuracy.

And all measurement methods require normal level of hydration, so rules for valid weigh-in day apply to BF% measurements too.
Were both measurements done on valid weigh-in days?

If 10 lbs really is fat and not potential inaccuracies in method use, then the following applies.

10 lbs x 3500 = 35000 / 90 days = 389 over true maintenance. Which true, I doubt.


Also, besides the almost daily 30 min bike ride that was unaccounted for, lay out your schedule for those other workouts for day of the week. And comment how intense the run is to you. Not the speed, that's meaningless, how intense for you. Same with weights.

So first, best BIA's are only up to 5% accurate if you are in the same hydrated state. So the inaccuracy overlaps each other there.
Calipers are same 5%, again overlap within the range of potential accuracy.
Best you can say is the following.
First measurements BIA and calipers:
22.5 - 27.5% and 23.5 - 28.5%
Second measurements:
26.5 - 31.5% and 27.5 - 32.5%

And you would have no clue which side of that best potential range of accuracy you fell for each set of measurements.
And if the accuracy was not at best 5%, but actually 7 or 10% - even more overlap.

So in essence, you don't have any measurements that truly tell a story that you can put much stock in right now.
But, if you were using the spreadsheet, that increase, real or not, of BF%, would cause your BMR to be lower, your TDEE to be lower, and your TDEG to be lower.
So you would eat less.
Self adjusting.

The only thing you know for sure is weight has gone up, you still don't have reliable enough data for BF% to know what it's doing yet. Those potentially inaccurate measurements give trends, over time. Like months.
2 data points does not a trend make.

So my last request to look at your workouts. You commented on the lifting, heavy for you.
What days of the week do those occur on, because doing certain workouts in certain order is just a stress on the body with little to no recovery.
And I asked for how intense the runs are, and those longer bike sessions, speed is worthless to describe intensity, you could be so fit for running or biking that the faster speed isn't intense anymore. Then again you could be totally out of breath feeling like you are going as hard as you can for each workout. I get the idea the interval run is hard.

Is the speed for running/biking now faster compared to those times in the past doing them? Since weight has gone up, same speed/pace would be bigger calorie burn. But if you slowed down a lot compared to start of year, you could be burning a lot less.

Why am I interested in this?
I still think your stress levels are jacking up your hormones and causing bad water weight gain.
Until you have a DEXA scan you have no clue if it's "10 lbs of pure fat".

http://www.bodyrecomposition.com/fat-loss/another-look-at-metabolic-damage.html#more-9313