What is the red stuff on my shirt collar?

I run a lot! I always wear a black turtleneck spandex shirt under my white Under Armour tee and after each 9 mile run there is a light reddish stain that builds up on the collar of the white tee. Anybody know what causes that? Is there a chemical in body sweat? I realize I may be opening this up for some "dirty" me jokes but I shower daily and my clothes are washed prior to going for runs...