What is a matter with my hamster?

I have a hamster named Lily. She is the most adorable dwarf hamster ever. About in late november, I noticed she had a little red bump on her back. I noticed he was BITING herself! In the beginning I thought it was a small itch that would blow over.... BOY WAS I WRONG! Today it is about as big as a candy light saver one her back. It is now hairless and swelling. She bight it a lot. Does she have a tumor? What's a matter with her? Please help me because vets are very expensive. Please please please answer this! I really need someone's help!