How many days can my 18 lb. Weenie dog go without food?

Our weenie dog stopped eating on Thursday. She will only drink water, sleep, sit on the couch, and occasionally pee. She threw up bigtime on Thursday and had diarhea that same day. Today is Sunday and she has not ate, pooped, or thrown up since Thursday. She is 11 yrs old. What could be wrong with her? Does it sound like poisoning of some type?