If your knees are like mine, that will happen when your quads get too tight. The hams and calves will then tighten up to try to compensate with the pull, and the end result is the knee gets pain as all the tightening pulls at it. Stretching well usually works for me, especially stretching the quad in a few different ways to try to get all of it.
Anyway, fantastic job reaching 120. Keep up the amazing work!