I get those too!! Except mine are only BELOW the shorts line (maybe a few stray ones at the level of the leg grippers). I've gotten them with cannondale shorts, because that's mainly what I had till recently. They have silicone grippers. But, most of them occur on the exposed skin.
The hives stay even after I cool down and shower like normal heat rash. Then they turn dark. Benadryl cream works wonders. The weird thing is that I've ridden in some major heat and had no problems. They ONLY showed up on rides loner than 60 miles. It doesn't itch.
My dad (a doctor but not dermatologist) said it looked like a photosensitive reaction to him. I don't get a sunburn. It's the weirdest thing.



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In the past, I have gotten exercise-induced urticaria, but an antihistamine pre-ride prevents it. I don't get EIU on road bike rides, even without an antihistamine. I will get it on my arms when mountain biking and on my legs when running. Weird.