When should I go to hospital for flu? My back and neck hurt. I've been vomiting with fever 101.9. And im weak and dizzy.

See MD/ER soon. Viral infections if left untreated, with vomiting and diarrhea could cause serious dehydration (and be the primary cause of weakness and dizziness). Adequate hydration at home is needed now, but if uncontrolled an md or er visit will provide for an anti-emetic and IV fluids. Good luck!
Now. It sounds like you are getting dehydrated. That will only make symptoms worse. You should be evaluated to see if you might need intravenous hydration.