Can you tell me what color her gums are? The same as the other pups?
How is her breathing?
Do you have a little thermometer? Does she feel warm or cold compared to the others?
Do you have any karo syrup, maple syrup, or nutrical?
When you say the gums are dark, are they dark pink or more of a purple?
What does the vomit look like?
Can you rub some karo syrup on her gums please to help prevent hypoglycemia or low blood sugar?
If the gums are purple she may have pneumonia or a problem with her heart. The purple tells me she is not getting enough oxygen around to her body parts.
The karo syrup will work in about 10 minutes.
You need to get her to an emergency vet for an exam and probably oxygen therapy.
I will be on here for an hour so we can wait an see. Try putting more karo syrup on the gums and monitor the color for a change. I have to recommend for the worst possible just in case.
I had a litter of pups once (my own) and the day before the last one was to be adopted he crashed and died despited my best vet emergency efforts. His liver values were 8 times normal so I suspect a liver shunt. He was perfectly fine until that day.