I've just started doing HIIT on the track instead of running on the treadmill, and oh boy is there a difference. I would run on 11 on the treadmill and it was becomming easy. I can barely take 15 seconds sprinting on the track.

My main problem recently is that after I do my first 2 intervals of 30 seconds each, I get huge cramps in the middle left and middle right of my back. I don't eat 3 hours prior to running. I also make sure I am properly hydrated. I also consider myself cardiovascularly fit, so I doubt it's a fitness problem. I just don't know how to stop these cramps, because sometimes they REALLY frickin hurt. Any suggestions on what could be causing them, or how I could stop them?