I had the same thing with my right knee and the Dr said it was a Bakers cyst. I put hot compresses on it and it finally went away, don't know what caused it, but it sure did hurt.
I am glad you do not have a clot as that was my first thought.
Have a seen a vascular doctor to rule out any circulation problems?
Have you tried a chiropractor? If you have any pinched nerves in your lower back, you will get those cramps and such.
I have the exact symptoms..pain behind left knee, stiffness/cramping in calf of left leg..painful to walk, but equally painful to sit. Standing up/stretching causes great pain. At times, shooting pain in front of lower leg and down sides of foot. Had Ultrasound yesterday...no blockages, no cyst, no clot. inflammation of tissues/unknown origin. This has been going on for a month now. Doctor gave me Celebrex-antianflamatory, leg exercises to stretch and recommended wrap around cold compresses. Your post was a year ago....I hope you are not still suffering-let me know. I'm counting on being normal soon!