With estimated TDEE around 2700, and eating levels lower to much lower during the past, and recent, times, I'd suggest stress is still present.

But meds do concern me.

Low thyroid issues usually cause you to be tired, less movement, they really don't slow the metabolism down that much, they effect the daily burn more if people aren't watchful and notice they aren't as active.
So it could be that outside exercise where you mentally get yourself going, and kids where you do too - you could be less active than you know, perhaps.
And therefore estimated TDEE is inflated above real TDEE.

Or your estimated TDEE is actually a bit higher if lifting for awhile, and not losing muscle during past diets. You could have more muscle mass than average for your stats, meaning your BMR and TDEE are higher. That would mean current and past eating level is really a bigger deficit than though, causing stress.

Or, actually less muscle mass than average because of past diets and not doing resistance training then. So BMR/TDEE is actually lower than expected. But I wouldn't expect by that much.

Or could be the meds and reason for them are stressing the body out, and stress causes water retention - upwards of 20 lbs from cortisol, usually carried around the stomach.
That stress along with too much deficit still and life, could be too much.

The water weight from lifting again would be expected, for repair and more carbs stored. 1 lb weekly realistic for couple weeks.

So several potential reasons for fast water weight gain.
Couple potential reasons for real fat gain.

Depends on which are more likely to be true - and that may indicate hold the course, lower cals because of inflated TDEE, or increase actually to remove stress from real higher TDEE.

Stream of thought above as they came to me. Probably left something out.