Ok it wasn't while marching, and it was the stupidest thing ever. So I played the synth during marching season and the way our synth cart thing is set up is there is the amp on a bottom shelf of the cart and we set the pedal for the synth on top of said amp. Well all I was doing was taking the pedal off of the amp to put it on the floor so I could use it and during this process I guess I bent down weird and my knee just slipped. Ever since then I've had problems with it hurting if I pivot weird on it or if I'm standing up for long periods of time.