Hi there,and welcome to BYC!
Have you checked her for mites or lice? Does she have feathers on her butt or is she bare? Is the butt swollen along with being red? Bugs can cause the feather loss or broken brittle feathers and make the skin red. Powder her DE or other mite killing permethrins. Other birds plucking her feathers along with picking at her butt will make the skin red as well. You would need to give the birds more room so they have other things to do other than pick on each other. Possibly add more protein to the diet so they aren't eating feathers. If the butt is red and swollen, along with diarrhea and a really stinky dirty butt, she could have vent gleet. A fungal infection of the vent. If this is he case, you can clean her butt, trim off some of the feathers and apply vaginal cream to the vent and the area around it. Put probiotics in the water for a couple of weeks to help cure the fungal/yeast infection.
Great to have you aboard and enjoy BYC!
X2
Alright and
