General

I've been intentionally gaining for the last 3 months..my appetite has increased a lot but every now and again I feel full very quickly after only eating a sandwich and I feel full for a day or two. What's going on?

A lot of people get like this after stuffing themselves like crazy. At some point, your body just needs time to process everything.

My guess is that your body has easily accessible calories, so you don't fell hungry because you already have what you need

Maybe eat like you normally would or slightly less, while still eating more calories than you need for those few days. Your body should adjust, right?

If you stuff yourself with 4000 kcal one day, that's about twice a regular person's baseline, so you might not feel hungry the next day, but that's just my theory