Knee Problem. Similar problems.

Dear Doctor.. I have pain in the back side(back portion) of my knee, (Like a pinch inside) The pain is since 3 months , but I thought it would recover on itself..but I hasn't.. Part-1 When I was injured , the first few weeks I felt a lot of pain on the top portion of 'knee cap'...It was very severe I couldn't walk... Part-2 Now, at present I don't have any pain in top portion of knee cap... But I have pain on the 'back portion' of the knee(L)...(pain is like pinch) Part-3 The pain starts only if I walk too much or , if I stand for too long , or if I try to straighten the knee while standing... (I can straighten the knee while sitting....) There is no pain when I squat. Part-4 I can feel some what 'rubbing' like sensation inside my injured knee , when I slightly move it while sitting... I already consulted 2 orthopedics but they couldn't tell anything... PLZ help, I hope d info is enough to tell what has happened, if u have diagnosed similar problems...