Can you give some more info (or a pic?). What sort of bulge; is it on one side, even on both, any scales sticking out, differently colored then the rest of the fish? Is the fish exhibiting any unusual behaviors?
I ask these questions, because (for example) it could be dropsy (a condition whereby the kidneys shut down and the fish retains water and gets all bloated). Its often a result of an infection elsewhere. Check out http://www.fish-disease.com/diseases.htm
and click on dropsy to see what the signs/symptoms are. See if you can match symptoms to any of the other diseases listed as well.