Gaining

I find that when I gain quickly for a short time (I just gained 8 lbs in a week) I get a hard ball belly, that is the fat all seems to go just to the inside of my belly. I can't continue gaining because the gut pressure gets too high (acid reflux problems). After a week or two, I start to feel softer in other places and the belly gets less hard. I can then hopefully get on with gaining again.

Does anyone else find this too?

People either gain weight inside the muscular system, or outside. It's genetic, and you can't change it. If you gain inside, it can damage your organs. You'll also look like you are pregnant.

I gain the same way, my belly bulges out and feels firm. I too have had issues with acid reflux, I'm on prescription medication for it.
I too have eased off my gaining for a while. That may change, given the holidays are approaching.