Well, my knees have never been perfect, as I tend to roll my knees in when I walk/run due to collapsing arches. I have orthotics in my shoes to correct that. They don't do anything when I'm cycling though. I think what really caused the pain though was when i partially dislocated my knee cap on a table at home. I worked on that knee for about a week and a half after that thinking it was just bruised. I think that's what really got it irritated/made it painful. It got sore one day and reached a point where I just could barely move it. That was when I took the time off. The fact that my knee still tracks weird just keeps aggravating it I guess. I went to a sports therapist who gave me stretches and exercises to help correct the wacky tracking of my knee but I guess it will take time to take effect. So I guess there wasn't one thing in particular that caused it, but whacking my knee really hard on that table is what really set things off.