HELP! My guinea pig has a lump?

If you got her at a pet store, take her back and have them look at it. They will do this for free. I would say maybe change her food and cage fillers. You can call vets around your town and they will give you estimates for checkups. Try finding a vet that will do a check up for free, or a pet store that will, so you can get that checked out just in case. Best of luck!