Bullying seems to be a fact of life with chickens. In my experience, the pecking order changes sometimes, and the bottom girl can move up. In a mixed coop, my Welsummer spent nearly the first year of her life with no feathers on her butt.
The next year, she was fine, and one of the Buckeyes was everyone's target. This year, it's the same six girls and no one gets picked on.
For the short term, if you don't have separate housing for her, can you separate her inside the coop? I have a Sapphire Gem who had a little problem with her bum, which is now bright red and featherless following a trip to the vet.
I don't want to have to re-integrate her with her coop mates, so I have a small animal cage -- probably meant for some kind of rodent -- that I put inside the coop for her. She's protected from anyone picking at her butt but is still part of the group.
Good luck with your girl.