This may be a simple case of "SWIM BLADDER DISORDER". A rather common betta problem that may be caused by overfeeding your betta.
Your Betta will likely recover on its own if you stop over feeding it. For the time being cut back on your feedings and see if the swollen area reduces.
Betta's with Swim Bladder Disorder are not contagious. Just cut back on the feeding for a while and the swollen belly should go away on it's own. If not, the culprit may be something other then Swim Bladder Disorder.