While I've enjoyed the other responses, the question is, is it bothering you? Because if it's not bothering you and keeping you up, just realize your body is keeping the blood flowing where it's really needed. If your brain were in your butt, it'd be a different story altogether.
I just went skiing and snowshoeing in N Vermont in daily weather of about 7 to 10F and wind chill to -11 to -15. Even when doing highly aerobic uphill, I'd be warm enough all over, but still have a cold butt and still feel a little cool in the legs.