Pet rat back problems/lump?

This is most likely a tumor. Both of my rats passed recently from tumors. They don't usually cause rats pain because 90% of them are belign. They may cause discomfort when the rat is becoming an elder and is sick, or what not. If it's not a tumor, its probably is her spine. This is caused by not using the back legs. You can go to the vet, make sure he's very reputable, though. I hope I helped.