something similar happened to me when i was running cross country. It started out as a dull pain and when i kept running on it, grew in intensity. In the offseason, i took up polevaulting, and never got my knee check out by a doctor. One day, I jumped on it, and it just popped out and back in. It was the greatest pain i have ever experienced. The doctor took an X-ray that showed my torn meniscus, and i had orthoscopic surgery a week later. His diagnosis was that the muscles on one side of my knee were just wearning down, so when a lot of pressure was continually being placed on my knee, it had a tendency to want to pop out to the side with the weaker muscles. He went in and tightened up the muscles on that side, and sewed my meniscus back up, and got all the fluid that had built up out of my knee. It was a very painful experience for a while, but my knee is a lot better. My advice would be to get it checked out and get an X-ray, cause even though it might seem like no big deal right now, if you keep working it that hard, it could deteriorate fast, with very painful consequences!!