cold weather

I've been riding in extreme cold weather (well below freezing) the last few times, and have noticed that my knees are sore, even after a short ride.

Does anybody else have experience with this? I am hoping that this is only a symptom of something I'm doing wrong, and not the state of health my knees are in.

I hope it's one of the following:
-not enough clothing
-improper bike setup
-simply too cold
-something else?

I really hope someone has an answer for me, because it would be tragic if I had to give MTB up!