Hi, welcome to the forum, the possibilities in your case are
gastroenteritis (often with diarrhea), a reaction to a medication,
lactose intolerance (difficulty digesting the lactose in milk), indigestion, gas formation, infection, obstruction etc. We can not diagnose the exact cause without medical findings.
You need to undergo stool examination and ultrasound to rule out the exact cause. Also take light food, maintain water balance as you may get dehydrated.