What Is The Fluid Filled In The Knee After An Injury?

I had a wreck about 10 years ago, among the injuries, I got a fluid filled place on my knee. The place where the fluid was and still is is numb. It feels like it has jelly in it. However, I have never had any problems with this until now. Over the past week, my knee has hurt me so bad. It is the same knee that has the fluid. The pain is on the inside of my left knee and hurts like hell when I bend it (to sit cross-legged). Can you please give me some idea as to what this is? p.s. I have tried aleve and ibuprofen. Needless to say they didn't work at all.