Diarrhea can really burn little baby butts.
My daughter developed the worst rash I'd ever seen. I took her to the doctor and it turned out she was allergic to disposables. I switched to cloth, but she rarely had a diaper on because I was so afraid of the rash returning. I aired her little hiney out more than she had a diaper. I put her in cotton training panties and put her on the potty. She was potty trained really early because she associated diapers with her tushy hurting and she wanted no part of that.
My son didn't get diaper rashes because I didn't have diapers on him all the time either. He also potty trained really early.
It's hard to say exactly what's causing your baby's rash. It could be a combination of things. Let her little tush air out as much as possible. Don't use baby wipes. Use a luke warm, soft wash cloth. After a bout of diarrhea, put her in the tub or rinse her off really well and let her get thoroughly dry before putting another diaper on.
Hopefully, it will clear up soon.
Best wishes.