I've had hives before... off and on some years for some reason
I've had to get pure benedryl injected into me every time I broke out... With a 20 guage needle >.< And it always made me pretty tired. With me, I've noticed that it's mostly with outside weather and how active I am. For instance, if it's spring time and there's a lot of pollen around, I'd be fine if I just sat down, but if I got up and got my blood flowing, then it would hit me like a brick.
Though, listen to Veronica, she knows her stuff