Find out if your sitter knows of anything that she might have gotten in to, trash, plants, treats. It may be that she fell and hurt herself. Either way I think you should get her to your vet to be checked out. If she is not vomitting or having diarrhea I would continue to offer her food and water. Make sure you support when going outside so she doesn't injure herself more by falling down. If she doesn't improve overnight and your vet is closed tomorrow try to find an emergency clinic, I don't think you should wait much longer.