Welcome to BYC! Please make yourself at home and we are here to help. Doesn't look like a tumor. Does she have lice, worms or mites? I'd treat for those. Was she recently broody? You may want to ask further here https://www.backyardchickens.com/f/10/emergencies-diseases-injuries-and-cures Best of luck!
x2 on the photos. Does she have any symptoms? Does it look like a puncture wound? Looks like her vent is gooped up a bit. I hate to do it, but sometimes I have to wash my chickens' butts! You don't want flies or bugs settling in there!
Glad you joined us! That doesn't look like a tumor to me, but I can't tell for sure from the photos. Looks somewhat like she has some mites--I would check for small, moving black or red specks on her skin and feathers. If you see any, dust with Sevin dust or use another mite/lice spray or dust. For more answers, I would post this in the Emergencies section of the forums. Good luck with her!
Welcome to BYC! We're glad to have you. It doesn't really look like a tumor to me, either. I agree with Bantamlover21, you could have a mite infestation going on there.