K.A.
hi it may be something a little bit more serious than just a diaper rash. 1 thing i would try first is A&D Ointment this works so great! I've used it on all 3 of my kids. and only 1 of them has every had a diaper rash and that was before i started using A&D my son is now 19 :)
if after a few days and you don't see a change take her to see her doctor you may need something medicated
good luck