Pain behind my knee when I sit to long. Also hurts when I try to bend it?

Baker's cyst. By far the most common cause for pain in this location is a benign baker's cyst. These sit between the hamstring muscle/ tendon group and the calf (gastrocsoleus) muscles. Most common cause iskneearthritis w/ a degenerative meniscus ( cartilage) tear. The tear can cause a rent in the knee joint capsule and allow joint fluid to escape into the back of the knee and form the cyst.