i have no way of confirming or disputing your "allergic reaction" diagnosis but i can tell you that sulfa drugs can have a very dramatic rash as a non-allergic side-effect and this rash can last days. an allergic rash is typically very itchy hives. a sulfa drug reaction common toward the end of your course of treatment does not usually itch.
i went to the er for a rash was given iv injection and ephinephrine went to dr. prednisone zyrtec benedry still rash. the dr. said i had an allergic reaction to sulfur antibiotic. i have been to the er and the dr. twice. i still have the rash it itches and burns. in addition to the iv treatment and shot of epinephrine at the er i have been on 30mg of prednisone benadryl zyrtec.