It sounds like the medial collateral ligament you injured. This ligament is attached to the medial Wikipedia reference-linkmeniscus (cartiledge). The signs of a tear to this are catching, locking and giving way.

So I would hazard a guess that you have meniscal damage as well as the ligament problems.

Agree with Jennifer that it sounds like you haven't had enough rehab to strengthen the muscles and thats waht you now need.