I can’t stop loosing weight!?

So my doctor thinks I might have ulcerative colitis or a stomach ulcer, we won’t be sure until I can get the endoscopy procedure done. But because of this, My stomach can only handle non processed foods, kinda like a paleo diet. I’m very limited to what I can eat. I don’t eat gluten or dairy, and only Whole Foods! This is the only way I can prevent a stomach ache.

Anyways, I’m loosing weight at a slowish pace. I don’t walk around much and I haven’t been active due to just feeling awful with my stomach problems. The fact that I’m sitting around all day and STILL loosing weight is troubling. I’m not anywhere near a dangerous weight at all, but I’m looking for a long term solution so I don’t get to that point. I loose about 3 - 5 pounds a month. (Currently I’m 5’6 and 132lbs, 22 years old). I use to be VERY! Active! I ran marathons and even placed in them, I played college soccer everyday, I miss that very much and would love to start working out again but I don’t want to risk loosing weight from it. Anyone have any ideas what i should do? I eat as much as I can without hurting my stomach:(