I have had knee pain way to long, Why?

I was in a minor dirt bike crash around 6-9 months ago there was swelling and tenderness for a few days but went away but not all the way and when i crouch down (bending @ the knees) it hurts pretty bad on a scale of one to ten 6-7 not the worst but very uncomfortable, and sometimes when i take a big step down or overextend it it almost brings me down. Any ideas?????