Try upping your water changes. I've lost really fat otos, as have others, maybe from internal parsites or just algae OD. Really fat does not= really good unless gravid & it's hard to tell. I hope yours is just chubby, I'm with feemia, uncomfortable.
One of mine looked just like that when it came to me from my sister's tank (still lokks like that). It was the only oto in there so definitely not gravid.