Help me, is this cancer on my rat?

it sounds like she has a tumor. This isnt unusual in rats. It may or may not be cancerous. A biopsy is really the only way to tell. If your rat is female it may be a mamory tumor. They happen a lot. Especially if your rat was bred prior to your getting her. A lot of pet stores get their rats in large uni bins and they may or may not have had babies before you got them. They can get pregnant as soon as 5 weeks old raise the babies in another 6 weeks and show no signs of nursing in another 2. When rats nurse young then dont breed again they become prone to gland tumors and forms of cancer. Please take your rat to a vet as soon as possible. It may not be painful now but it can become painful quickly.