What Causes Knee Pain After Surgery?

I'm 19, 5'10'' and about 250 lbs. Today I was running on a treadmill on my offday for baseball and while I was running, my left knee popped, and now I cannot straighten it while I walk and the outside of my knee is where all of my pain is. While sitting with my knee straight in front of me, I cannot lift it in the air. I tore my acl and meniscus 5 years ago and had reconstructive surgery and just last year had my meniscus removed.