Sudden sharp pain inner knee when straightening leg

I did a brief yoga move that I do every morning (downward facing dog, hips swinging to the side while bending same side knee and keep heel on floor..stretches the achilles). As I was walking out of the room, I got a sharp pain in my right inner knee. It hurts when I straighten the leg and put pressure on it. If I walk with a the knee slightly bent and carefully, there is no pain. There is no swelling, I can bend, squat fine. Any idea what might have happened?