Injury to the bum
Looks like feather picking.
Is she on the bottom of the pecking order?
What type of feed do you use, what treats do you give?
Have you check her over for mites/lice?
How big is your coop/run?
How many chickens do you have?
Where are located, state/country what are your temperatures and weather conditions?
Keep her isolated so that will heal for a day or two, the others will pick at the red.
If found a couple of articles for you about different reasons for feather picking.
Some of the main reasons that might apply for you could be, depending on how may extras they get-they may need a higher protein ration or less treats, you have a bully-space is a bit small - roosting is at a premium - winter months everyone seems to get on each others nerves more, so adequate room to get away from each other can help. You may have to separate RIR for a couple of days, then put her back in the flock (hopefully will lower her status). Sometimes you may have to repeat.
For you SLW care, it's great you notice this, it could have gotten really ugly. It should start to heal fairly quickly, keep an eye out for infection (I know you are ).
thanks for the suggestions and the websites. I tried a few ideas and it seems to be working. A few more girls were found with the same symptoms, just much more mild. After washing them and putting a good coat of Vaseline on em, they seemed to have lost interest in each other's fluffy hind quarters. I also put in some boredom busters. Keeping a watch, but hopefully this'll work. Thanks again for the suggestions.
Glad it seems to be working!
I'll have to remember vaseline.
Thanks for the update.