A lot of things happen in and around the knee when you exercise, especially on rough terrain. Rather than speculate, I'd suggest that you see a good sports medicine physician, or an orthopedist. They have all sorts of diagnostic tools at their disposal(X-ray,MRI,CAT,etc) and have spent years studying exactly this kind of situation. If you try to self diagnose, friend diagnose, or just tough it out, you run the risk of incurring serious damage. The road back from a seriously damaged knee is long and arduous, and ofter fails to achieve 100% of original function. Best of luck.