Re: unusual seiza problem - pain in BACK of knee
If you have intense pain you stop immediately. Nothing helps like a break.
Get any medical advice you can, talk to the sensei before or after you do so, it's nothing bad, He/she wouldn't disagree.
When the pains go away, you can think about doing a limited return to those kinds of practice, your sensei can get you working in a 3 instead of a two, missing every second round...something like that so you're only doing 1/6 of it.
I don't like those techniques, apart from kokyu-dosa.