Nate – thanks for sharing your experience. I am having similar symptoms after a tough PT session – very tight muscles behind my knee and difficulty bending it, even after I had pretty good flexibility before, i.e., ~ 135 degree bend.
It’s now very tight and painful. I have been using calf and hamstring stretches to try to mitigate, and breath through the tension.
The stretching is helping a little, but both ice and heat have brought some relief as well.