What are these non-itchy red spots on my legs?

Everytime i go running (about 3 miles a day, 3 days a week) about ten minutes in i develop a rash on my legs. Mostly on the front, above the knees. It is like little 3-5 mm circles dispersed randomly. Not raised or bumpy. Does not itch at all. I don't think it's environmental, because it happens no matter where i choose to run, and those places are about 10 miles apart. I am a 29 yr old female in fantastic shape. 5 ft 2 in., 116 lbs.