How Can Popping Sensation In The Knee Along With Intermittent Pain Be Treated?

A few months ago, my left knee popped. Now, if I m not careful it will hurt behind the knee and down the back of my leg if I bend it either lying down or sitting. Sometimes there is pain in one spot below the knee sometimes in the front sometimes in the back. The pain also causes me to limp. When it is hurting I can only lift my leg partially.