It's urates in feces scalding her rear end causing redness, swelling and irritation. She is most likely rubbing her chest on the roost at night. Wrap an old towel around the roost.
The Nu-Stock will heal both areas.
Apply the Nu-Stock as mentioned by @Wyorp Rock in the above thread. Shake the tube well and wear disposable gloves when applying it. It will heal affected areas and prevent picking and pecking.