What Causes Knee Pain With Numbness?

I am a 47 year old male and about three weeks ago I started having a sharp tearing tyoe pain on my outer left knee. It also causes a constant numbness at the same location. When I am on my feet for a while or walk moderitly the back of my knee will hurt. There was no injury to my knee and no crusing or noticable swelling. Any advice for cause and treatment Doc?