Mine does the same thing. He does not always eat everyday. Shih tzus are very picky dogs. Avoid feeding them table scraps and that should help. He could go several days without food, but he will eventually eat. Dogs will not starve themselves to death. Try adding a little bit of canned dog meat (like Caesars for small dogs) or add a little bit of low fat, plain yogurt (unless he's allergic to dairy). Shih tzus are also smaller and so their energy runs out more quickly. If he's not eating, then he's not having enough calories and so he'll sleep more also. Puppies also sleep more then adults too.
It's not really possibly to give a definitive why without more information and an exam but you should see your vet and probably today if possible. Those don't mean one thing but often mean something is wrong and sometimes very, very wrong. Your vet should be able to tell you more after an exam and being able to talk to you about things and ask questions. I hope your baby is feeling better very soon.
How long has your shih tzbeen experiencing these symptoms? Does she have diarrhea? Do you know of her getting into something she should not have? Not eating or drinking much is pretty serious. I would suggest you get her to the vet as soon as you can before the issue gets any worse.