What Causes Itchy Rashes While On Bactrim For Staph Infection?

my daughter is 14. Took Bactrim for 5 days for a Staph infection. On day 5, rash and itchiness began, along with low grade temp. Stopped antibiotic. Started benedryl, zantac and steroids. Rash does not seem better (now 48 hours later). At what point should I seek emergency treatment? No soa.