What Causes Bruising From Knee To Buttocks After An Injury?

Hit my knee cap really hard on the dash board of my car over a week ago hurt really bad at the time. Forgot about the injury until a about a week later then started having issues with knee popping & feeling like it was going to buckle on me. 2 or 3 days later noticed had large area of bruising on back of leg from back of knee to buttocks. Also feel lump behind knee in leg.