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Chances are it isn't a cancer or tumor. Your best course of action is to see a vet that specializes in small animals. Many that are more inline with dogs and cats may not have all the experience and skills you are looking for. I also found a very good article that might help you. The link is attached below. All the best!
Hiii...Generally owners may be unaware of their rabbit having most forms of cancer as rabbits are prey animals. This means that they are programmed not to show signs of pain because, in the wild, this makes them more at risk from predators. Depending on where the malignant tumour is situated, the signs and symptoms can vary. At the vet, the rabbit suspected of cancer is X-rayed and the possibly infected tissues biopsied.