it seems that you have some kind of injury in the knee, by this mean that there appear to be a deep muscle pull at the back of the knee or perhaps an internal injury inside the joint.
Have you had nay injury or trauma at the site in the past?
I think some local application may be tried with perhaps with some bandaging on the top to restrict the movements for a few days. OTC painkillers are another option. If still not relived you may see a doctor for a further assesment
The most important cause of pain behind the knee is baker’s cyst. Baker’s cyst can arise from a cartilage tear.
The diagnosis can be confirmed by ultrasound and examination. Mostly the cysts can be treated by rest and limb elevation. A surgical excision can be done if the cyst is causing too much discomfort.
Please discuss this possibility with your orthopedic specialist and keep us posted!