Maybe you have a combination of things going. Both myself and one of my other friends also get this hiking in hot weather. For us, it's just extended walking in hot weather that does it.
As a golfer who has had this problem over a number of years, I cannot disagree that it is heat related, however I believe that chemicals on both the fairways and particularly in bunkers are the main cause. When playing in short pants, I notice it immediately I play a bunker shot, but only if the sand has a particular chemical.
I am Irish, in my mid-sixty living in the UK and playing golf all over the world.
I haven't noticed any correlation to hydration as I'm always very well hydrated.
I've just had my first case of it this year, and as always, it came on a hot day on the golf course. I started noticing it on the 12th hole. My first symptom is a tingling sensation and "fullness" on the back of my calves.
I know two other women golfers with this condition and happened to be playing with one that day. She realized she was getting it on the 17th hole.
I don't have any pain associated with it other than if I touch it. For example, I can't sit with my legs crossed, and I'm very gentle when toweling off after a shower.
I've found this forum after a lot of searching after trouble with this condition.
I find I get the rash if I wear walking boots on a longish walk (8m plus) on a day with temps above about 18C.
Similar to what some have said I find the rash very debilitating, it spreads mainly above my socks (a little below but not much) up my calves. Red like bad sunburn at it's worst lower down then blotchy further up. It feels hot and like bad sunburn and the skin feels tight. It is also very painful to walk.
Unsightly I can cope with but the last couple of miles when it's bad are very slow and painful!
I've found that I don't get it anywhere near as bad if I wear very light walking trainers with breathable walking socks that claim to keep your feet cool.
I'm pleased to find others with the problem but like you surprised that the Dr's don't seem to know. I did go to a chemist in Ireland when it was at it's worst, they'd never seen anything like it either!
I'm also fair skinned, welsh with Irish descent.
I live in Hong Kong and I have had "golfer's vasculitis" since I moved here two years ago. What I have observed, is that when I am well hydrated -- drinking about 64 oz of water every day (and have done so for several days), it seems to prevent the rash. Last summer, I was very careful to make sure I was always well hydrated. I never got the rash if I was. This summer, my hydration habits have not been so great, and the rash is back. Let me know if that makes a difference to you.
I've read about 100 of these posts - my word they are numerous! I haven't read anything about preventing the condition. I have tried coating the legs with Vaseline BEFORE the walk. Twice, I have not had the rash. Is this circumstantial? Have I missed a post about prevention (without pills)?
BTW, I get the condition in the dead of winter.