That is scald from urates in feces. You'll need to clean her rear end more often or trim back feathers and fluff so that she'll have a clear "shot" to the ground when she poops. Do not trim them back too close, just a little.
Apply Nu-Stock to the area and the redness will clear up in no time...
It looks as though she may have had a minor prolapse and was able to retrieve the exposed tissue. In case there are fissures or cracks in the reddened skin, I recommend cleansing the area with 50/50 betadine and water mixture, pat dry and then apply neosporin. Then Keep an eye on the area and...