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I'm sorry to hear that your little dog is feeling poorly this morning.
My concern with your dog is that she may have injured his back at some point yesterday and it wasn't until a little time had passed before the inflammation caused her to be so painful. This is actually very common with dogs and back injuries. As long as they keep moving, they feel okay, but once they rest it seems that they stiffen up and become more painful (sort of like how if you have a sprained ankle, it always hurts worse first thing in the morning when you step out of bed, than it does after you've been walking on it for a bit).
If this were my dog, I would get her to an emergency clinic in your area today so that they can take an x-ray of his back to make sure that there's no fracture or slipped disc (which is very common in long backed dogs like Shih Tzus and in more fragile breeds such as Italian Greyhounds and Chinese Cresteds), and get her started on some anti-inflammatories and pain medications to help her feel more comfortable.
If you don't have access to an emergency vet today, I would do your best to keep her quiet and comfortable until tomorrow morning, when you can call your own vet. If she won't walk, then you can pick her up gently to take her outside to urinate and defecate, but then right back inside and kept still if possible. If she's used to being in a crate, you may want to layer it with soft blankets or towels and place her there until you can see your vet in the morning.
I would absolutely make sure that she gets seen by a vet as soon as possible though, as if this is a back injury and it's left untreated, it could get worse to the point she's unable to walk.
I hope she does okay!