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.
- Gray 2010 carbon WSD road bike, Rivet Independence saddle
- Red hardtail 26" aluminum mountain bike, Bontrager Evoke WSD saddle
- Royal blue 2018 aluminum gravel bike, Rivet Pearl saddle
Gone but not forgotten:
- Silver 2003 aluminum road bike
- Two awesome worn out Juliana saddles