Argh. I went running in cold midwinter evening rain (10*C), and actually had a good time getting slightly lost in Mt Cook. However, when I came back into the house, I suddenly found I had heat rashes all over me. I think I'm doing something wrong...
Edit - this is apparently known as exercise-induced urticaria. (Thanks Reddit!) It has to do with the natural release of histamines into my system when I exercise, as these inflate my blood vessels in a helpful way - unfortunately, I then have a reaction to the histamines. Which causes an allergic-type rash. Blah.
There are several things you can do to alleviate that.
1: Don't get wet.
2: Wear clothing that doesn't cling when it gets wet
3: wear clothing that won't rub (causing chafing)
Running outside is still fun. I am continually amazed at how distances that seem like too much bother when walking seem trivial when running.
On the other hand, my friend's gym is offering him free trials to give to his friends, so I am going to try the indoor life for a bit!
I'm glad you found the cause of your trouble. Is there a solution to it as well?