Hi my Grandson is 20 months old, and the last couple of days he is breaking out in hives only when he sleeps, he could sleep in the bed or have a little nap on the couch it doesn't matter, he has hives on his legs and arms when he wakes up, his mom gives him Benadryl and it goes away, but we are woundering what is causing those hives only when he sleeps. thank you