My betta had a almost growth looking thing of his stomache. What's wrong with him? Or is it his shape?

My betta had a almost growth looking thing of his stomache. What's wrong with him? Or is it his shape? It is the same color as him and doesn't seem to change his behavior (very active), but could this become dangerous? (He is in a ten gallon with 3 other bettas with dividers, heated water, and a filter) I just want to know if I should seperate him so the others don't get sick... if he is sick. Thanks!