How To Stop Vomiting?

hi my daughter was just sitting on the couch on Saturday and out of no where just started throwing up. she has been fine and laughing and playing. she was fine all day Sunday and eating laughing and playing again. when I got home tonight I went outside for a smoke and she was crying. I thought she was having a temper tantrum. when I got in she was covered in vomit. why is she doing this