This did happen to me once when I was in school. We had to wear small shorts and on that particular day I had borrowed a pair from a guy in another period because I had forgotten mine. After a few laps aound the track, I had that same feeling you mentioned. Before I knew it, I was blowing a load in my buddy's shorts. I couldn't stop and it ended leaking out on my legs. Works part of the whole thing was that the showers were broken that day and the water was out. My buddy needed his shorts back for the next class so I wiped them and myself off the best I could and let him have them. I never told anyone of this and made sure to adjust myself better the next time we had to run
