When I significantly increase my running pace, I get a pain in my chest/side. By "significantly increase", I mean running 2 miles at a 7:00ish minute pace within a 5 - 7 total mile run at my normal 8:00 - 8:30 pace. While running at the accelerated pace, my breathing definitely gets heavier, but I do not hit my breathing maximum. I breath harder for short periods while swimming or weight lifting.

The pain feels like a pulled muscle or dislocated rib. The pain lasts for a few days. The pain is located on the left side of my body, centered about 3 inches below my arm pit. It hurts worse when I twist my torso from from side-to-side, especially when I twist to the left.

Does anybody else experience this phenomenon? Does anybody have a guess as to what is happening to me?