What Causes Pain And Swelling Behind Knee Cap After Twisting

I was on slanted hilly ground and turned wrong and kinda fell on one leg and twisted my knee, the pain is behind the knee cap, there has been swelling and pain, and its been over a week now, and I have almost twisted it two or three more times just walking and doing normal activities. It seems to twist side ways and I can feel the crunching feeling again. I went to the mall today and after a good while walking there I felt a stinging down towards my ankle, the knee got stiff, and then my hip started hurting, and I could not walk another step. When I arrived back home I look at the top of my ankle and you could see like blood vessel brusing on top of my foot near my ankle, and the knee is extremely stiff again, and slightly swolen again.