I see you've put this in the Skin
Cancer forum, is that because you really think you got skin
cancer on a trail run, or just because there is no "nettles" forum or "poison ivy" forum or "mystery rash from plants" forum? That would be my first guess. It could also be that someone was spraying weeds, and your skin got a nice dose of 2-4-D. Take a good shower with decent soap, and see if things don't calm down by tomorrow.