The same thing happened with my beta last year at school. He was always very active and funny, but suddenly became very bloated. At first he didn't seem to mind it or seem in pain, and he had been eating normally. I cut back on his feeding and it didn't seem to make a difference.
After trying to find the problem online, I tried to feed him peas, which he refused. Eventually he began to lose scales around his bloated belly. I felt so bad for him, but eventually he did pass away. Poor little guy, my roomie and I miss him. My sympathies to you both. <3