Seeing that this is a girl she can have a couple of things going on she can have a bacterial infection, or a yeast infection or even a food allergy. I would call you doctor and describe it to him even email him/her a picture of the rash so they can see if they really want to see the baby or just phone in a script.
My daughter gets Yeast infections alot, but My son had a food allergy that looked the same. My oldest sone had the worst rashes when teathing.
Zink oxide is the best for healing skin so the original desitine extra stength has worked everytime for regular rashes, if a yeast infection Nystantin, if bacterial an antibiotic will be needed.
I would call, see what they say (pediatrician)