J.S.
If you call your pediatrician there should be a doctor on call that can answer your questions and either agree to see her right away or direct you on how to treat her until Monday. You shouldn't just assume that the doctor is closed or won't take calls on the weekend.
The reason is that rashes can be tricky. It could be an allergy, but it could be a bacterial infection like ringworm or it could be a virus or something else, all things that need immediate treatment... all which need different types of treatment.
The cream, whatever it was, obviously didn't work so don't use that again. Since the rash got worse instead of better you really need to call the doctor.