What Causes Locking Of The Knee?

I fell on my concrete porch a couple of months ago on my left knee. It swelled and bruised for about a week and then the swelling, bruise and pain went away. Then about a month later my knee would lock and pop every time I bent it. I would also experience a severe pain everytime I moved my knee whether bending or straightening. Now I if I squat down I have to slowly get up on my right knee and baby my left knee because if weight is put on it, it feels as if it is going to give out. I am a healthy 26 year old, however I was diagnosed at 9 or 10 to have juvenile arthritis in my knees. What could possibly be my problem with my knee?