G.B.
Wash her off each and every time with clear water to get rid of the stomach acid that is on her hiney. Then take antacid liquid and dab it on her hiney with a cotton ball. This kills the acid in the pooh and off her skin.
I was told to do this for the kids in my child care center when they had severe rashes that were due to diarrhea, not for any other reason like a heat rash or yeast infection, and it would take care of it. It did.