What Causes Pain And Stiffness In Knee After An Injury?

Hi, I was in a car accident about 3 months ago and my knee cap got pretty banged up. I went to a doctor and they ruled out any breaks and just said it was bruised and to ice it. That was fine for then, but it's 3 months later and still hurting and sometimes becomes very stiff. What could this be, if not a break or bruise?