I will second that it seems like bloat from over eating. Try feeding less when you do feed. Bettas are pretty well known for being over-eaters. I would also try switching food. I have had some experience in the past with some getting too bloaty on flakes.
So much chronic bloat may indicate an underlying issue with the betta's digestive system, but the best thing you can do in that case is to do your best to avoid too much bloating, and keep the water parameters as good as you can. Putting a single, shelled, blanched pea into the the feeding rotation may help too.
Two roads diverged in a yellow wood, but thankfully they both led to fish stores.