I'm having pain behind my left knee on the side too, can't bend or straiten well weight bearing is tolerable with assitance?

See an orthopedist: Sounds like you have a meniscal tear or cartilage injury.. Some of your symptoms are typical - including pain and locking. Other symptoms may include swelling, clicking, and instability. Check in with a local orthopedic surgeon for an evaluation. He will probably recommend physical therapy initially and perhaps an MRI depending on you examination. Arhtroscopic surgery may be necessary.