Winter is very hard on warm water fish, like Bluegill, so it is not unusual to loose a couple going into spring. Probably 99% of fish diseases are secondary in nature. Disease organisms are present in every pond...they just do not cause a disease unless there is an oportunity, like a stressed out fish.
Don't worry about one or two ill looking fish. However, if you see dozens, then something is wrong. That's when you need to trouble shoot.
