Check for a UTI. Otherwise, she may have reached an age where she doesn't feel she wants to be uncomfortable, and she knows it is cold outside. For the next while I think I would take her out in the mornings rather than allow her to take the initiative via the doggie door.
Different situation, but perhaps a relatable example, my Border Collie was absolutely obedient his whole life, until the age of 14, when he decided that remaining in the field with me on a down stay when the day was hot wasn't to his liking, and he opted to return to the shade by the house. I think that as they age dogs sometimes decide that their comfort is more important than obedience. Or it could just be the onset of senility.