Just my two cents worth here, but I also took my daughter to the doctor for diaper rash. She gets a pretty raw butt and it seems to coincide with when teeth are coming in.
We were give an anti fungal (Nystatin ?) and a steroid to mix together and use twice a day. The other times my doctor told us to use something with Zinc Oxide in it and use enough so we cannot even see her skin through it.
Also, we started giving her daily baths (was every 2 or 3 days because of dry skin) and that seems to help.