There is a possibility that the fish might be pregnant and is soon laying eggs. It could also be due to an allergic reaction. It would be best to have a vet look at the fish to ensure that the fish is in a good health condition.
also beware that that is a sign of illness that can lead to death. A pet store that specializes in fish should have people who can look at it and tell you if it is fat, pregnant or ill, and they will do it cheaper than a vet.