Usually I rely on rollerblading or nice brisk walks around the city for exercise. Every once in a while, though, I get the urge to lace up my running shoes and hit the road. I’m able to keep up a decent pace without getting winded and all that, but for a long time I’ve noticed something odd: after a while, I get a weird sharp pain in my right shoulder (below the collarbone and just to the left of the joint). This happens without fail after maybe ten minutes, and stays steady until I’m cooled down, at which point it disappears. The weirdest part is that it never shows up during other activities–even if I’m exerting myself to a roughly equivalent (or even greater) degree.
Anybody got a clue what causes this, and why it only shows up in these specific circumstances?