Unfortunately this sounds like dropsy, which has a poor prognosis for a cure. Dropsy isn't exactly a disease, but a symptom of another problem. It is usually caused when the fish begins retaining too many fluids due to kidney shutdown.
Many times this is caused by an internal infection. A quarantine tank is advised while treating the fish. Cures are not reliable with dropsy. The only one I know of is to feed the fish an antibiotic food and treat the water with a medication for gram-negative bacterial infections. A salt or Epsom salt bath can help draw the extra fluids out of the fish and bring the swelling down. Add 2-1/2 to 3 tsp/gallon of water.
This is usually caused from a water quality issue or stress to the fish that lowers their immunity to disease. I believe that this is a likely culprit in your case as you don't know your water parameters. Do a 50% PWC immediately.
Like DragonFish said, you need to get an API test kit to keep an eye on your water parameters. If your ammonia reads anything above .25ppm or nitrates are above 20ppm, you have a problem. NO amount of ammonia is acceptable, but .25 ppm is barely acceptable for short periods.