A few thoughts. You have to consider what is behind the knee. There are blood vessels, tendons, muscles, knee joint. You have to consider each of these. When you are evaluated these will be looked at to try to clarify your symptoms.
Patellofemoral. Often times, pain from the patellofemoral joint (kneecap) is felt behind the knee. Additionally, the seated position often bends the knee. Bending the knee causes increased pressure on the kneecap, potentially making the pain worse. Another possibility is if there is fluid in the knee, it can cause pain in the back. There are many causes of fluid. See a doc if it doesn't get better soon.