Cat Health Questions? Ask a Cat Vet for Answers ASAP.
If she has been vomiting and not eating or drinking for that long, she is probably very dehydrated by now. The head shaking may be a sign that she is feeling weak. We can also see cats have weakness and problems holding their head up if they have very low potassium levels, which can occur due to kidney disease, not eating, and various other causes.
Definitely get to your vet as soon as you can. If they take after-hours emergencies, you would be safer going in tonight though it's hard for me to know if this is absolutely necessary without seeing her. They will probably want to do blood work, and may want to take an x-ray of her abdomen depending on what they find on their physical exam.
For now, try to get her to drink some if you can, and make sure she is warm. Cats who are severely ill and dehydrated will often get hypothermic.
Good luck, and I hope this all turns out OK!