If it hurts when she pees, it may be thrush. If so use daktarin cream (which is an antifungal) about 3 times a day. Wash with water between each nappy change (and dry), and try to get lots of nappy free time.
You can use daktozin, but if you use cloth nappies, it has zinc oxide in it, so it may coat the nappies and render them water proof!!

We are big nappy rash people here - I have had all sorts of nasty teething rash with Jenna. She flares up every time she is cutting new teeth - and her sores are seriously nasty.