I'm having chest pain after a run. Help?

I'm 16. Although I'm not much of a runner, I can run pretty fast. In school, we have to run a warm-up lap on the track around the football field; I usually sprint half of the lap and then run steadily the last half. I know running really fast can cause chest pain afterward but what i don't get is that how come I don't receive any chest pain when I sprint but when I run steadily the whole lap. The chest pain lasts for like 7 min. During those 7 min, I can't do anything. And, I always drink a quarter of normal size water bottle abt 20 min before the run. So I don't know what's going on. This happens every single time I run steadily during a run. Now, it's been at least 11 hrs since I run. and I'm still having a chest pain... help?