Any popping and clicking in your knee? if so where? Does it lock up?
does it give way?
Have you ever fallen over?
Can you feel any firm feeling mass in the area of pain or anywhere on the back of your knee?
Have you twisted and feel something "tweak"?
Has your knee swelled up significantly ever or at all? Have you banged your knee on anything lately?
Have you Hyper extended it?
Answer those please.