Is Ceftin Recommended For Strep?

Our 4-yr. old tested negative for strep yesterday, (very red throat plus fever) but swab wasn't thorough, so in case, the Dr. prescribed a Ceftin drug which he didn't start today due to intolerance/throwing it up. What has been out of control is the rash accompanying the sore throat/fever. Covering most of his body- very red, splotchy, some bumps...but so itchy he is going crazy! Worse at night of course. Have eliminated bedding and other likely variables/causes, so have assumed it's some kind of side symptom of his infection. Nothing brings relief-- Childrens Benadryl, cold compresses, prescription cortisone ointments-- so we're at wit's end for him and us. Any ideas?