rash-like redness of legs after running

I have many of the symptoms listed above. I typically only get this "rash" on the lower part of my leg (left) right on the muscle and above my calf. I rarely experience on my right leg for whatever reason. I began wearing compression socks when running distances over 5 miles or so and it literally stops the rash from occurring. I also have used a topical 1% hydrocortisone cream after returning from a run and that of course relieves any post run itching.

Although, I can see this area getting red as a run it doesn't really itch to the point of distracting me as I run.
If I have the time I have found that a quick massage of that area pre run can also help. This has bothered me very much the last 6 months or so and although this discussion is kind of old I was glad to find that others are experiencing this and providing some answers.

I have gotten this heat rash since I was a child - I am now 28. After years of trying to explain that I couldn't finish the required 2-mile run each week at school and having it be completely dismissed by the teachers, I've gone to a doctor and had it confirmed. For some reason, my skin just doesn't sweat normally - this happens randomly and has become such a nuisance as I've begun running 5Ks and want to eventually get to marathons. Heat rash has definitely caused me to reconsider my exercise regimen, as the itch becomes extremely unbearable. It begins as a prickly itch and gets increasingly worse if I continue movement at all. It's usually only on the tops of my legs, sometimes on my feet and ankles, as well as my stomach. I can do miles on the elliptical just fine without irritation, but any actual running where there's impact seems to trigger it. I've found that unless I exercise very regularly, the onset doesn't improve. I've taken to keeping Benedryl on me during races to keep the rash at bay :-(