I'd say either an allergic reaction to something or might be shingles. I got shingles almost a year ago (thankfully it wasn't major but it looked like that - started behind my ear and worked it's way down my neck and to my chest and it was itchy - Apparently it can be caused by stress and can be contagious with skin to skin contact of the infected area). My son just had an allergic reaction to penicillin and it started off looking like that too and by day 4 he was one big hive. I say try some Benadryl like Other Mummy said and if that doesn't help I'd go to the doc just to be safe.