How old is she? Was she rolling around naked, swimsuit, summer clothing?
Most of the time it is hard for Dr. to diagnose a particular rash and the allergen that caused it. If the outbreak is not too bad, they usually tell you to take benedryl and use Cortaid crème.
I think an oatmeal or Aveeno soothing bath might help along with the OTC.... HOWEVER... since you do not know the cause of the allergic reaction, I would FIRST call my pedi and get the recommendation from him/her.
My hubby is very allergic to Poison Ivy.. he doesn't have to touch it to get it. So far this summer he has been at the Dr. once for Steriods to help clear it up which helped a lot with the rash on his face and eyes. We live on a wooded lot and try to watch for it but he has had it over a month now on various areas of his body.
I wash sheets and clothing almost daily as well as making sure dogs are washed because it can come in on them as well. It is weird, I can rub it all over me and not have 1 reaction but it has put him in the hospital.
Good luck and like someone else said, make sure you watch her carefully for any sudden swelling, shortness of breath, etc. If you see that go to the ER.