R.A.
Could it be the change in diapers? I know that happend to my son once. My oldest had a rash once that was so bad he was bleeding! Took him to the doctor and they told me to use milk of magnesia (sp?) and aquafor, either dab it with a cotton ball or you can mix them both together and put it on like a cream. I love triple paste, however I've also noticed that butt paste sometimes helps. After she is done with her bath keep here bottom out to air dry ( I let my kids go around being naked for a bit) Also they say when they have irritation to limit wipe use. So when it was bad for my kids I wouldnt use wipes for pee and wet paper towel for poop. What kind of wipes are you using? Are you using wipes at all?
Poor girl, hopefully this clears up soon, and good luck!