What is wrong with my 3 month old teacup chiuaua puppy?

She is usually very active but starting around noon Saturday she began to not eat or drink and is acting weak and lethargic. She was tested for worms today and it was negative and her shots are up to date. She has thrown up puppy milk that we tried to give her to help since she wasnt eating and also thrown up yellow fluid (bile). Her ears are always back, tail between her leg and only wants to lay down. Any information will help!