I like Knott's idea. But I have a few other thoughts as well.

Where exactly in the pedal stroke are you feeling the pain? Example: starts at around 1 o'clock and lasts to 10. Since we're talking about your left leg your pedal rotation will be counter-clockwise (12, 11, 10....)

Are you able to keep your knee cap aligned over the second toe when you do your 1 legged squats and heel together squats? Can you use a plumb bob to check?