I need Hamster Help!!!?

My hamster Gwen is a little under a year {{I got her last March}} But as soon as we got her, I noticed her tummy getting bare. She was chewing off her fur. I switched the bedding, and left her in the care of someone else as I had left for a couple of months, well I come back and notice that is has gotten worse!! She looks like an 80 yr old hamster! She's got patches of fur missing and her body looks so skinny and thin! I don't know what what to do and I didn't get a cat or dog because I couldn't afford to take them to the vet, and a hamster is inexpensive, but I feel so bad and I don't know if she's in pain or not, but I can't bare to see her like this. HELP! please I need some advice on what to do!