Hey Twistersmom, I suspect that your fish has a tumor. Even though you only noticed it today, since your goldie is the round-bodied type, it's easy to miss a gradual swelling if you don't sit down and examine him every single day. If you're just seeing him at feeding time and only for a couple of minutes, you might miss something swelling up like that. It's likely that since you were fiddling with the filter and spending a lot of time at the tank, he may have had this swelling for a while but you just happened to notice it then.
Tumors are common among goldfish and they are usually benign unless they grow too large and begin to press on internal organs. They can be removed through surgery, but costs can range from multiple hundreds of dollars to completely free, depending on fish vets in your area. Sometimes veterinarians will do the surgery for little or no cost since the chance to operate on a fish is relatively rare and can be a good learning opportunity. Again, depends on your area and vets.
As long as your fish is acting normally and feeding well, there's absolutely no reason to do surgery other than his lumpy appearance.
If you don't mind and he seems healthy, leave him be. But if he stops eating or acts listless, best to take him in for a checkup.