What Cause Chronic Knee Pains?

Hi im 20 years old and a female and have been experiencing pain in my knees, mainly my right one, since i was around 14. They begin to hurt only after standing for about 30 minutes to an hour and when i cross or bend my legs they really hurt and feel almost like they re stuck. It is like an achy feeling. I ve noticed when i walk, my right foot points outward while the left one is more straight. I think i walk like this because it helps with the discomfort, because when i try to walk with both feet straight it doesn t feel right..almost like one leg is longer than the other. I went to see the doctor about this when i was around 15, and don t remember what they told me was the problem but they didn t treat it like anything serious and said I would grow out of it. So it has been years and I haven t grown out of it and don t think I will. I just would like to get an idea of what the problem is. I m currently 180 pounds, 5 foot 6. From some research I have been doing it could be arthiritis or knock knee(genu valgum), but I really don t know though. thanks