It sounds like a mammary tumor....which are often non-cancerous and are successfully removed with ease.
It's definitely worth taking her in to the vet to check it out. The vet can do a biopsy and help you decide what to do. Rats are very stoic, and you often cannot tell whether they are in pain or not.
If the reason for not being able to do surgery has to do with the cost, you should consider checking with some vets, or your local humane society, because there are many kind and caring people out there who are ready to help anyone and any critter in need.
I bet with a little snooping, you could work something out that would allow your rat to get the surgery for low, or no, cost. I've even heard of great vets allowing someone to work off the cost of their rat's surgery by helping in their clinic.
Many rats live very happily for a long time after a tumor removal surgery, so for the sake of having more precious time with your friend, it's worth checking into all your options!
