BMSRN is right. I also had the exact same problem during football my senior year in high school. Went to the chiropractor, and he told me the same thing. My problem was hamstring tightness, so I did alot of toe touches, and quad holds, and it felt better in about two days. But you really have to stretch your hammies. Hold the stretch for about 20 seconds, then strecth farther, and hold for another twenty seconds, and repeat until they feel really loose.