You also have a male Yorkie who is not neutered. How can you be positive that he did not breed with her? Were they together at the dog sitter? Regardless of who the sire of this possibly pregnancy might be she needs to be spayed because of her size. It just would never be safe for her to have a litter of puppies. While you are at it you really should have your male neutered so that you do not have any more problems like this in the future.
__________________ “Never argue with a fool, onlookers may not be able to tell the difference.”  Mark Twain |