I Have Pain In My Left Knee. On The Outside.

I have pain in my left knee. On the outside. Feels like I hit it on something. I was quite painful early on, then it eased up, now the pain is back. Hard to put wieght on it. Taking Ibuprophen, and Tylanol which helps somewhat.

What could be causing this discomfort? I have not been diagnosed with any arthritis, still active.