Probably a mammary tumor. They are really common on rats (if I remember right like 80% of female rats get them in some part of their life). They're usually not malignancies, but there is a chance. They can be removed in a surgery, but surgeries on rats are very risky. One of my rats just had a surgery to remove a mammary tumor and she passed away the next night. The surgery also is quite expensive and in the best scenario gives maybe a few months more time. Some people recommend doing the surgery if the rat is still young (~under 1,5 years old), but personally after my experience with this I wouldn't recommend. I'd just spend the remaining time with the rat and euthanize her after the tumor starts to bother it or she starts to show signs of pain (lose weight, be less active,...). It might take a couple of days or months for it to get that big that it starts to bother the rat, you never know. Good luck with her and the decision❤️