My legs are very sore after sitting for a while. the back leg (behind the knees) hurts and i can't straighten my legs for a few seconds, then its fine.?

Knee pains: This sounds like degenerative arthritis of the knees, where the inflammation feels worse after you've been sitting for a while, and slowly gets better as you walk it out. If the pain is especially intense in the back of the knee, you might want to get checked for a "baker's cyst", which could be removed. Other problems could be: torn cartilage, inflammation of the knee cap, bursitis, even a clot.