It sounds like you might have a rotational issue. If your leg is not completely straight, it will wear and cause pain on one side or the other.
A simple analogy is your bike has a chain and two sprockets. If one sprocket is out of line with the other, the chain will wear severely or jump off of the sprocket. Same principal for the knee. If you are not completely aligned from your hip to your foot, you will mechanically wear out the joint (sprocket) in the middle or your knee.
There is a lot in here on rotational issues and the Moderator when she looks at your post will most likely give you suggestions. Keep reading the material here for your condition and let me know how you make out.
I would most certainly get an appointment with an OS and ask if he/she is concerned with your alignment.