bumping for you.....

When you were eating "around 2100 calories" - were you actually logging to know that - or rough guessing it?

And was eating 1600 the same method?

Do you actually weigh the foods that go in your mouth, and that includes packaged food for servings?

And beyond that, unexplained weight gain, which doesn't happen when you eat less than prior maintaining at - is reason to see Dr.

Elevated cortisol from stress on body can retain 10-20 lbs.
And doesn't have to be stress you are aware of, and unrelated to diet or stress thinking about weight gain.

Sadly you'll really have to think back to any other changes you remember from back then.
Like why did you feel bloated - what had you eaten? Had it again?

They tested RMR actually, which is 150-250 above BMR.

And 150 below what was expected for gender, age, weight, height could be bad, then again if you have less muscle mass than average, it could be right on.
Then again it could be suppressed because of prior dieting attempts, even to eating 2100, which would sound high, but may be low.

Got a good bodyfat % estimate, or several not-so-good estimates to average together?

That will give a better idea if RMR was close or not using Cunningham RMR formula.

And you are correct, if potential accuracy was the same between eating levels, that doesn't explain it. So you could easily have been eating more than that in both cases, or if you tend to inflate "just in case", you could be eating well less than that.

If you think it's stress, then you just gotta get out of it. Start measuring and find past measurements to see if really any increases to be concerned about, or perhaps you have clothes that say there is not because they still fit well.

If you don't, you can keep yourself gaining, and cutting out more calories will just increase stress on the body beyond your mental.

It doesn't sound like you have the normal symptoms of the real metabolic damage, so what's left is the stress.
http://www.bodyrecomposition.com/fat-loss/another-look-at-metabolic-damage.html#more-9313

RMR - you came in and sat down and a breath mask was hooked up to your face for about 15 min.
BMR - you stayed overnight in lab and they woke you and put a breath capture hood over your head and you went back to sleep.

And yes - you most certainly can lower your RMR by eating too little. But as I said, it can also be lower than expected because you burned off muscle mass, but it may then be totally correct for the amount you have left. Hence having a BF%.
And 25% is healthy for a woman.
It's not at the athletic level. You a training athlete?

Were you honestly logging your calories during the slow creep up in weight?

Everyone that claims they gain on 1200 calories also admits they binge and have cheat days and meals - and while the goal may indeed have been 1200, their actual average eaten was higher.
Throw in inaccurate food logging you have, even higher.
Do that on a suppressed metabolism - you can easily gain fat, because each pig out day is actual surplus of food, and body is going to store it as fat in case the craziness continues.

So you know the definition of insanity? Reread your description of what you saw and what you did regarding weight and calories.

Hopefully at this point you realize goal weight is a range - not a number. All kinds of totally expected reasons for it to go up - as water weight.
In fact your BMR literally does change through the month, for you, no decisions on weight going up or down should really occur until you have a month of data, and then another month to compare to.

Yes, you probably do need more calories than you think, and more than you've been providing.
Sounds like for good long stretches you were eating an easy 50% less than your body could have been burning.

Get the spreadsheet on my profile page.
Stay on the Simple Setup and Progress tabs.
Look at the sample data, change it to see effects if desired, then delete all the yellow cell data.
Put in your own stats and measurements.
Get your planned activity in there to see potential TDEE.

From what you are eating now, add 100 calories daily for a week at a time, making your way up to that TDEE.
You can hang at the TDEG level for couple weeks to see if no response, but then keep moving on up.

If by eating a mere 100 calories extra daily for a week, which would take 35 days to actually gain 1 lb slowly (reread that), but you gain more or faster, then you just confirmed how much of a deficit you are in.
Just topped off water stores with storing more carbs, because you were very depleted likely.
That will probably happen the 2nd week too when you increase another 100 daily.

Correct on changes to bodyfat while weight stays the same. That only happens if other improvements cause weight gain, like muscle mass. But that requires more energy.
If your body has already suppressed your systems to conserve energy to better match the food level going in - is it going to make something new, (which takes more energy to make), which causes daily increase of energy? No.

Ya, you have no clue how much you eat until you weigh and compare. Even packaged items with "about 2 servings" is going to be off for so many reasons.
Fresh items that are any size you'd have no idea really.

And what can work at the start with huge margin of error won't work later on when it's much smaller.

So you could be like average person and are actually eating much more than you think, which means prior you really were too.
Or you could be vastly underestimating the amount, because you don't really know and trying to play it safe, and creating a much bigger deficit than you know.

Insanity - doing the same thing over and over expecting a different result.
You said "I wanted to stop gaining weight, so I ate about 500 cals less, but my weight didn't drop. It would stay stable for a while and then creep back up, causing me to drop my calories further. Then I would remain stable at this higher weight for a while, until it would creep back up again (without changing intake or exercise) and I would lower my cals a little again. This has happened a couple of times, leaving me with the lowest caloric intake in years, but my highest weight ever. I'm at a higher weight than when I started to lose weight, but I'm watching what I eat much more, having less calories than back then and exercising more. "

You can't use any formula's or do any math frankly - one whole side of your equation is unknown - so it really doesn't matter what the other side says.

You can reread my last post last paragraph for recap on what gaining faster than fat gain means and why.

Like 3 lbs in 3 weeks, you are correct, IF, IF that was fat, would mean 500 more daily than maintenance. But was it literally 1 lb a week, or more faster in 1st week, than nothing last 2 weeks, or you only weighed start and finish of 3 weeks so no idea how fast or slow the gain was?

You should do the 2 week test. Perhaps unstress you too.
Eat 250 more than current eating level.
If you were indeed already eating at true potential full burning metabolism maintenance - you would gain 1 lb slowly over that 2 weeks.
If you are actually eating at suppressed metabolism maintenance - you will potentially gain more faster or nothing.

And this needs to be a real measured 250 calories daily - not some wild guess of weight and calories in whatever you eat. So find and weigh something that will give that many extra calories. Perhaps a glass of chocolate milk measured or something, just simple.
Keep your other logging as sloppy as it is.