That sounds an awful lot like dropsy to me. If that is the case, its a condition caused by a secondary nature - such as a bacterial infection. The kidneys become infected which cause the fish to not be able to release toxins from its system. The body starts to balloon and will eventually "explode" through their abdomen. Dropsy itself is not contagious, but if it is caused by an internal infection, the bacteria can infect other fish and make the 'condition' contagious.
Medications like Metronidazole (Emtryl), and Clout are usually your best chance at fighting Dropsy (or Bloat as it is sometimes called.) Interpret #9 Anti-Bacterial or Octizon can be used to treat the infection as well if you can't find the first two. You can also do salt baths with the fish to help release the toxins. Add 2 Tbs. per gallon of water (same temp and parameters of course) and let the fish swim in it for no more than 30 minutes one to two times per day. Remove them at the first sign of stress. This creates a diuretic effect. If caught early enough, you can cure the infection before organ damage occurs. Once the kidneys have been affected, the fish usually will not recuperate well, but can soetimes survive. If they shut down completely, they will die.