Good question. I always feel the same way after Christmas, and I tend to hibernate for a bit. It's not unusual. I have felt a bit out of it this week with Thanksgiving, because it was busy for several weeks beforehand, and then my kids had Wednesday off too, and then it was just so nice not to have to go anywhere. But I have things to do, I just don't have motivation.
Getting out and taking a walk in the sunshine helps me get re-engergized, or at least gets my mind focusing in a different direction.