Cyst on my guinea pig...worries?

I have a 2-year-old female guinea pig who has a small (pea-sized) cyst on her back. She's had it for several months, and I don't think it's gotten any bigger. Otherwise, she is a healthy, happy, pleasantly plump piggy. I am a poor college student so I don't want to have to pay any unnecessary vet bills, but I also want to take good care of her. She shares a large, open cage with another 2-year-old female guinea pig who appears to be in perfect health, and their diet consists mainly of guinea pig pellets. Any thoughts or suggestions?