Knee Pain

For about a year now, everytime I don't move my leg for about 15 minutes, my knee starts to hurt, and the pain intensifies. Once it started to hurt, I can move it all I want, it won't matter. It started with the left knee, and now, about two months ago, the right started to do the same thing. Anyone have any ideas what it could be? I mean, for my knee to hurt, when it doesn't move? (it's really anoying having to move my legs all the time, just because it'll hurt if I don't... And when i'm driving, I don't really have any space to move them, so I usually end up where i'm going, in pain.)

anyone?

Thanks,
alyson