Has this fish not always been this shape? Some are meant to look like this :-?
If it used to be a common, thinner shaped fish, then something is definitely wrong.
It is hard to tell from the photo but are the scales on the fish all sticking out like a pinecone? By this I mean everywhere, not just around the fat bit. If so, it is possibly dropsy and the fish is unlikely to survive.
If it is just the belly which has scales sticking out, then it is possibly constipated. Try feeding a cooked, shelled pea. If that doesn't work, or the fish won't eat it, Google for treatment with a bath of Epsom Salts.
It may also be egg bound, if indeed it is female. Not sure what to do in that case unless you are willing to try and gently push down from head to tail in an attempt to expel some eggs.