Baby getting over flu..yellow vinegar smelling poo?

My baby is getting over a wicked flu she has had for the past 5 days. She was sick with a high fever, diarrhea and projectile vomiting. The fever is gone, she is no longer vomiting and is in generally great moods but her poop has been changing from very watery green to yellow and vinegar/chicken soup smelling, despite the fact that she has not been eating either! She is formula fed. Does anyone know why it could smell like vinegar?