Since your puppy hasn't been fully vaccinated for Parvo, it is possible that she is infected with this virus which causes lethargy, vomiting, lack of appetite, and progresses to bloody diarrhea, dehydration and death.
Other than Parvo virus, there are other viruses and bacteria that can causes illness in a young dog. The other concern in a puppy is that she could have chewed up or partially eaten something that is causing a stomach upset; if she vomits everything she eats, then an intestinal obstruction is a possibility. This is usually diagnosed via an Xray (sometimes with barium).
At this poini, it is very important to get food and fluids into her. This is the time to try a good quality canned puppy food with her. Alternatively you can give her some cooked, chicken meat (without skin and bones) and cooked white rice in equal amounts. This is a bland, easy to digest diet.
Since chihuahuas are such small dogs, they are more likely to become dehydrated in a short amount of time (within 24 hours) so you can also give her warmed-up chicken broth or chicken noodle soup (which has important electrolytes in it). The other thing you can give her is Pedilyte (infant electrolye solution, found at most drug/grocery stores). Use the unflavored variety and use an eyedropper or something similar to give it to her by mouth. She probably won't drink it on her own.
It's also helpful to take her temperature (using a rectal thermomter); normal temp in a dog is 100-102.5 F so anything over 103F is a fever. If she is running a fever and continues not to eat, then it's probably a good idea to have her checked out by your vet on Monday.