The child had diarrhea four times three days ago and then developed a fever. After examination, it was found to be a viral infection, and the fever has subsided. There was only one episode of diarrhea yesterday, and the doctor advised to observe. If the fever recurs, hospitalization may be necessary. This afternoon, after eating formula milk, the child vomited. At night, the child vomited after two more meals, with no other symptoms. What could this be?