Hives from biking?

Anyone get hives when biking long?

I rode my first century a few weeks ago and got hives on my upper thighs (below my shorts, though).  Every ride since this (7 rides), more hives.  They are not hugely uncomfortable (just annoying) but it's very odd.  So far, I've been able to eliminate my during-ride drink(s) and nutrition and different pairs of shorts as the problem because it happens every ride and I've eliminated at least one of them on every ride.

The best I can find from the internet is that some people get hives from exercising and from sunlight.  I've never had this problem before, despite living my whole life in Georgia and riding/running regularly in the dead heat of summer here. 

I'm concerned because they seem to be getting worse with each ride.  Yesterday, they moved up from my ankles to my outside calves and I think they might be on my scalp (it's really itchy).

Any thoughts?

I took some Claratin this mroning and will continue this week to see if perhaps a systemic antihistimine will help.  Benadryl helps a little, but knocks my out and makes me dehydrated (bad for biking in the heat).