Betta floating oddly

I had a Cambodian Betta who was getting a little old. To be more accurate he
was coming up on 3 years. I always felt bad for making him fight in his
earlier life so i took extremely good care of him. He was gigantic as far as
males go and a bit of an overeater.
Recently he began acting strangly though. He would stay on the bottom most
of the time, darting at top speed for the surface when he wanted air. It
seemed that he had lost his ability to float of hover in his bowl. Within a
couple days this disorder prograssed so that he was floating nose up near
the surface but still alive. I decided that he had seen better days and put
him out to pasture so to speak.
Does anyone have an idea what was wrong with him? I don't want this to
happen to my other Bettas. It was not bad water or any kind of unhealthy
enviroment at all. It was strictly a sickness or disorder but I just have
never seen that hppen before. Please fill me in.