This might be due to intestinal parasites or it could be she is starting with a viral infection.
With such a young and small dog you may want to consider seeing a vet tonight. Young pups don't have a lot of reserves and small ones can run out of energy even faster than usual.
You can see if she can handle a drop of Karo or pancake syrup on her tongue for energy. You can see if she can hold down a little plain pedialyte. But a vet can give her fluids and an antinausea shot and start her on meds if this a viral or bacterial infection.
Hope this helps you!
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