Possible causes for the pain in my left knee...?

Two years ago, I was play fightinh on the stairs with my brother (bad idea, I know), any way I kicked him &he grabbed my right leg causing me to fall down three stairs onto my left knee. It hurt very bad! But, I'd hurt my knee many times and figured it would heal on it's own. Last year I went to the doctor for it, he claimed 'it's normal', whatever that means. Anyway, another year later I'm still in pain. It's worse in the winter, worse when my leg is between being straight &being bent at a 90 degree angle . It doesn't hurt when bent closely together (knee to tight). Any ideas?