Toddler Hives

So for the first time ever, my 3yr old woke up with hives down his backside and on his face. I sent him to school and told the teacher to kee an eye on him. She said they looked fine throughout the day but once he woke from his name they were all over, worse then this morning. He hasn't eaten anything different, no new detergent, soap etc., doesn't seem to have been bitten by anything, not on any meds... The school nurse gave him Benadryl, but I'm freaking out because this has never happened and k don't know why it's happening... Any ideas????