keep an eye on it and monitor its growth and her energy levels. most lumps are benign but they can grow to a size that could cause problems. you should discuss with your vet your best option when dealing with them. soe don't grow too large and though the rat won't be winning any beauty contests but will be fine. having said that though that is not always the case, it is possible that she could have cancer. if that's the case you'll notice soon enough if she goes off food or starts to lose weight even though she's eating lots. unfortunately there isn't a lot taht can be done for the malignant tumors as often by the time they're noticable the cancer has spread to other parts of the body. some steriods can be used but i just wait for the rat to tell me on her own when she's ready to go and don't normally bother with the steroids. however it COULD be a simple abcess which antibiotics and perhaps draining will fix right up. without seeing it myself i can't tell you which it is. all you can do in the meantime is watch and wait. i hope all goes well.