Is this normal? I figure it's from the blood being pumped harder, but it lasts for about an hour. I get really itchy, too.

I have terrible environmental allergies and get hives frequently.

During a visit with my allergist, she said that hives can be triggered, or exaserbated, by all sorts of things...exercise being one of them.

Are the red spots raised and itchy? If so, perhaps that is what it is.

ya, it's normal. your skin reaction to what ever your sweating out your pores
or clothes rubbing on your skin. try not to wear cotton when you workout, it retain moisture.

I immediately thought hives too...

Sounds like hives. Either just plain exercise induced or possibly a low level allergy to your detergent that really only gets exacerbated when your pores open wide.

I would also look in to wearing only "technical" wicking fabrics to keep the skin as dry as possible.

i know sometimes i'll get a itchy bump or two around my back after working out. seems like bug bites to me, personally.