My DS had a very similar thing a couple of weeks ago after having his first lot of antibiotics for an ear infection. Like you I was worried it might be a reaction to the AB's (or german measles) so took him back to the doctor. She told me that it was to do with the virus he'd had and it was his body having a reaction to it which was a good sign cos it showed that his immune system was over coming it. She also said that if he was going to have an alergic reaction to the AB's it would break out in the first day or two, not after four or five days. Hope this helps.

ETA. Meant to say that his rash disappeared the next day, from start to finish it was probably only there about 36 hours.