What Causes Pain And Popping Sensation On The Back Of Knee?

My knee is in a lot of pain. I feel almost a popping sensation on the backside of my knee but it doesn t feel like bone popping it feels like muscle or tissue. almost like plucking a rubberband. It hurts to move my knee when it s completely straight it is fine and when it is bent and stationary it is fine but moving between the two positions hurts. What is the problem?