Random knee pain after sports injury?

I received a hard blow to my knee in High School Basketball (I am 28 now) in which I had a Large goose egg for several days above my knee cap and couldn't walk on it.  Ever since receiving the injury my knee will occasionally (usually after using stairs or just walking) go out.  Completely stop supporting me, sometimes I fall because I go to step on it and my leg just crumbles under me.  And then when I try to walk on it I get a shooting pain inside my knee that feels like someone is amputating my leg (happens at least once a month).  Otherwise my knee is fine and does not hurt at all.  Does anyone know what I could of done to it?