Is your fish just young? and is it a fancy goldfish? Is it a bump all over its head, the same colour, as if the skin of the head had just got a lot thicker? if so, many goldfish species have this naturally. if however your fish is not young, or the bump is uneven, then it may be something else. I suggest you take it to the vet, get a book about goldfish, or do some reading on the web (depending on how much you love the fish and how rich you are!) : fishy farmacy has a good diagnosis tool on its website
http://www.fishyfarmacy.com/symptoms.html