Could be lots of things!
Is the whole fish bloated? And if so, are the scales standing out? That would be dropsy, and you need to treat the fish with oral antibiotics ASAP!
If it's only parts of the body that are swollen, it could either be internal tumors or possibly some internal parasites/worms in the body cavity. For this, I would treat with medicated food against parasites/worms
But like Hanna said, it'd be good if you'd post a pic! And I would also want you to test your water's pH, ammonia, nitrite and nitrate levels!!