What Causes Rashes And Hives On The Extremities And Trunk?

My toddler has a rash w hives on his extremeties and trunk. He was on amox q 5 d as treated for L&R Otitis Media. Possible for this to be viral on top of the infection he has or maybe a reaction the the antibiotic? I did D/C the amox today but new hives have formed. I also wonder if it may be a heat/cold rash that some toddlers develop in hot/cold climates like ours. Sorry I am a gastroenterologist, hard to remember those ER rounds and Peds/FM rotations. Would like some thoughts... Thanks in Advance...S. Andrews M.D.