HELP!!!!!! Dog Health Issue?

My springer spaniel (6 yrs old) has a small bump under his skin on his belly. It is soft and can move around, and it doesnt seem to bother him no matter what i do to it. Its diameter is probably the size of a quarter, and it's "squishy". I hope its not a tumor please tell me if it is so i dont freak out before i take him to the vet (which i will so dont just tell me to do that plz). Thanks!