What Causes Episodes Of Small Hives In Child?

Hi, my son is 4yrs old and for a week now has had episodes of small hives on his face and randomly on his body. They dissapear as fast as they apear!! so strange.By the time i go grab the calamine lotion they are gone. Im suspecting from warmth because its usually when hes laying down,or wakes from a nap. He was on amoxicilin 2weeks ago for an ear infection,but the Dr said its most likely not the culprit. There is no fever or any other symptoms. So strange. I am monitoring everything he does and eats to see if there is a pattern. Hmmmm.....????? sugestions? Thanks! Tracy