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Hello from JustAnswer.
The vitamin B12 injections are not reported to cause vomiting and diarrhea, so these symptoms are most likely related to a viral infection of the gut, which is the most common cause of acute vomiting and diarrhea. Although you do not specify the flu symptoms, headache and congestion of the upper airways is reported as possible side effects. But since she is also having the vomiting and diarrhea, the flu symptoms are also likely due to a viral infection.
Since she cannot keep anything down, it would be better to limit her oral intake to clear liquids.
If I can provide any additional information, please let me know.
You are most welcome. I hope that she feels better quickly.