Help, terrible chest pain while running?

I'm training with my high school for cross country over the summer. Ever since I've started running, there has been a terrible chest pain 2 1/2-3 inches from the bottom of my rib cage, kind of near the heart. My doctor thinks it is a side sitch, but it is far too high for that because if I ever get a side stitch, it is near the bottom of my rib cage, and side stitches are like cramps, and this feels like something really heavy is sitting on my lungs, not allowing me to get the maximum oxygen I need. It feels like something is getting blocked off in my lungs when I breathe.