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!
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!