Hmm sounds like it could be many things such as:
*Dropsy /OR/
*Constipation /OR/
*Pregnant Fish /OR/
*Tumor
More than likely, its not a tumor. I only say this because they are more rare than other diseases/ailments.
Try adding some aquarium salt based upon the recommendations on the box of salt. Also, make sure the water is clean. By clean, I'm talking partial water changes every week, and weekly tests of Nitrates and possibly Ammonia...pH can also tell a lot.