Stevens johnsons sydrome?

Is this a rare disorder?? All I know is that it causes severe blisters and burning of the skin. My daughter is on bactrim antibiotic (low dose) for her kidney reflux and I dont want to sound overly worried but I noticed her skin was a little red ? And she has a red patch on the back of her neck(it looks streaky)and red spots ontop of her head (which Ive actually noticed 2 months ago, but didnt think anything, thought its a heat rash or something) butShe started the bactrim on wednesday. and today I noticed the redness.. I dunno. should I be worried about SJS?? Does bactrim really cause it?? Shes getting it looked at wednesday along with her shots, but until than im going to be a worried mess!
Thanks