Re: unusual seiza problem - pain in BACK of knee
If the pain is in the back side of the knee and not just behind the knee cap it could be anything from compression of soft tissue structures to a Baker's cyst. Baker's cysts sometimes accompany rheumatoid arthritis and some people just have them-I have a small one in one knee. Other considerations could be vascular issues-cutting off circulation to the tissue, aneurysm.
If the pain is more behind the knee cap, the I would consider chondromalacia patella which is softening and roughening of the cartilage on the back side of the knee cap. You may want to get it looked at if the pain is excruciating.