Exactly where are the hives? Are they in spots where the shorts cause the most pressure, like around the leg grippers and under any seams?

I had several allergic reactions to food last year that manifested as a kind of combination between hives and dermatographia. The problems with dermatographia lingered for several months afterwards.

http://www.mayoclinic.com/health/Dermatographia/DS00755

Basically I would get red, itchy, inflamed skin -- sometimes distinct red circles, other times just big patches of red skin -- from a combination of pressure and seams or wrinkles in my clothes. And the rashes usually appeared after the pressure was gone. For example, I would be lying on my side while sleeping, then turn over, and then the skin on the side I had just been lying on would start to itch where the pajama seams were, or where the fabric was wrinkled and creased under me. And there would be red marks from the seams/wrinkles.

Scratching made it worse, and made the rash spread. Antihistamines did help.

It has mostly gone away now.