My thoughts are wandering the same direction as TsPoet's - could it be heat/sweat. I've only had hives once in my life and it was from heat in a sauna. I broke out in honest to goodness big itchy hives on the tops of my feet, behind my knees and in the crooks of my elbows after having spent some time in one.... It took a couple of hours for the hives to show up and then the next morning.... I had a beautiful reaction on my face - all around my eyes was all swollen up, I looked scary....