@fify-- I agree with you! I think the temporary rash you talked about is called "viral rash." That's what my daughter's pediatrician called it.
My daughter was using penicillin for an infection in her tonsils and she developed a rash and blisters towards the end of the treatment. The doctor said that she doesn't have an allergy to penicillin, just a viral rash. But he still took her off the penicillin right away.
I guess viral rash is not a big problem. But from what I understand, it's still not okay to keep using penicillin in that situation. Especially if the rash spreads and forms blisters.