There are so many things this could be. For example, it could be a blockage. It could also be a heart problem - like a faulty valve that causes low blood pressure and then blood serum can leak into the abdominal cavity and swell. It could also be a liver problem which ca also cause swelling. It could be an abscess. No matter what though you need to get to a vet. Doing nothing and hoping it will go away is dangerous.