Is this her first litter?
Is she shaking?
Can you check her gums and tell me if they are pink and moist or pale and tacky?
Is today the first day she is off her food since the start of her pregnancy?
Any discharge from the vulva?
Providing she is not showing any symptoms, I wouldn't be overly concerned but if she is still refusing to eat by tomorrow I would have her checked out. Try offering her some jarred meat baby food (make sure it does not contain onions or garlic) These small breeds can become hypoglycemic pretty easily, as I am sure you know, so you can mix a bit of plain pedialyte in with the baby food and give it to her via needless syringe or dropper. I have had girls that go through this for a day or two throughout the pregnancy..my vet states it could be hormonal..but they started eating and acting completely normal within a day or two. :) If she starts to show symptoms of lethargy, fever over 103 and has vomiting or diarrhea, do seek veterinary care. Good luck to you and your future momma :)