Pain in my leg after running...?

I'm a sprinter, and I've been training for over a month now. I always stretch before and after practice. Lately I've been getting pain on the inside of my lower left leg (like my shin). I've done some research, and I don't think it's shin splints. I'm fine when I'm running, but when I'm standing waiting to go again, that's when it really hurts. I've never had an injury to either of my legs or ankles. What is it?