When I was 10 I was very active; I played many sports. Eventually one of my teachers noticed I was limping and suggested to my parents I go get an x-ray. The x-ray showed up positive for cancer, which later was debunked as a spiral fracture in my tibia.
Ever since this incident, however, I've had problems with my knee. I have extreme tenderness on the outside of my left knee, so if I press into the skin any there the pain is overwhelming and I can no longer move my joint with ease(it gets stiff.) I also began to notice that I could no longer sit on my knees, because if I did my knee felt like it was stuck together. It locks up whenever I do something similar to sitting on my knees(crouching, even.) The only two ways I've found to get it out from its locked position are just forcing it out or taking my time and gradually massaging it out. Both methods provide a very loud pop.
I've been wondering what it is for a very long time, but when I went to a doctor last he just told me it was arthritis and sent me on my way without an explanation(this was when I was 13, I believe.) Could it possibly be a tear in some tissues in my knee? It also just randomly hurts throughout the day, some days it hurts much worse than others. I've never noticed any swelling.