Hives

I can't figure this out Wednesday night Carson came to my bed because he was scared. So I took him back down to his bed and laid with him until he went to sleep. Hours later he was so wiggly and wouldn't stop moving. Next morning I go to put him in the tub and he is covered in HIVES! I felt horrible that is why he kept moving all night. I gave him some benadryl and they would go away for a couple hours and come back. Now it's Friday and his last dose was before bed last night. I'm hoping were done with these nasty things. This is only the third time in his life that he has had them and I am clueless to what causes them. They look a lot worse than they really are, this is the first time he's had them that he is old enough to scratch them so his body looks like he was attacked by a cat. The pic above is the type of hives Carson has.