Re: unusual seiza problem - pain in BACK of knee
My guess is that your lower body is currently in a very unnatural state. Your tissues are far to tight and your joints are not free to move as they should. Sadly, this is all too common.
The good news is that with commitment and effort, I bet you can make significant changes in three months to a year.
Some ideas: Start foam rolling, learn proper deep squats, and work on hip and ankle mobility (you can find numerous videos of all these on YouTube). And let go. We create the tension in our bodies.