J.P.
Give her lots of naked time to help with the redness. You can also put something on her bum to create a barrier between the skin and poop, like some rash cream. Something oily seems to work better, even something like bag balm. The diarrhea could be teething (mine is the same age, and same thing). To help with that you can get some infant probiotics. BioGaia has some drops.