Why do I keep having pains in my right knee ?!

Ok... For the last couple months Iv'e been having pains right below my right knee... It goes away after like two or three days and comes back a few weeks later... I finally decided to make an appointment with my doctor because the pian gets very uncombfortable particularly when I'm lying down in bed and of course it hurts when I walk... Does anyone have some suggestions as to what it could be I have heard of somethings it might be but want more opinions before my appointment next week.

Ps: I haven't done anything to stress it so not sure what the issue is.