Body adjusting from under-eating to "normal" calorie intake?

I don’t want to bore y’all with my health issues, but I genuinely am interested in how people have dealt with their bodies adjusting to a pretty significant change in calorie intake.. surely I can't be the only one.

Here it goes:
I have severe GERD. Blah blah blah, rant about legit health issues, blah blah blah. Basically, for a short while I have had to under-eat due to worsening symptoms (mainly vomiting). Maybe 900-1100 calories per day...this is NOT by choice. I know it’s unhealthy. I've been on some new medications and surprisingly they've been helping me hold food down. I haven’t been able to eat MORE food yet…but the food I have been able to eat has stayed in my body like it should. I've had a slight gain and that concerns me a little because I've had fairly steady weight loss and have lost 50lbs the last 21 weeks.

I’m worried that once I’m able to hold all my food down, and eat the amount of calories that I should be eating, that even though I’ll be working and continuing to eat healthy…that I’m going to gain some of that weight back until my body gets back to “normal” and learns how to function “normally”.

I guess my question is this: How long did it take you for your body to adjust to eating normal? And if you gained weight..how much did you gain?

I’m trying to mentally prepare myself in case I do end up gaining. (hopefully not a lot) but I won’t let it slow down my motivation. I still have another 50lbs I want to lose. I’m a firm believer in slow and steady, so even if it takes me longer than I intended to get to my goal, at least I’m still getting closer.

Thanks for your time.