What Causes Foul Smelling Stools,vomiting And Fever In Infants?

Baby 13 months old for about 2 weeks has had huge amounts of foul smelling stools during day and nights. The week before this started she was throwing up. Has also had fever,but that is gone. Also lethargic. Some days eats a lot,some days not much. Stopped breast feeding at 12 months. She drinks soy milk.