My knees are stiff, due to unknown reasons. I've been in and out of the doctors, and gotten 3 surgeries on the knee, 2 on an ankle. So I have indeed been going to the doctor for this, and they haven't found reason, but they have been giving medication for the problem, but they don't do very much effect towards my knees.
What sort of physical activities do you participate in?
the only physical activity I do is
walk 30 minutes to school
and 30 minutes back home.
Try Googling some exercises/stretches for the legs. Sometimes your knees can get stiff from inactivity and can get pretty bad from sitting cross-legged or sitting in a chair for long periods at a time. Especially if you have had surgery on them. They need to be stretched.