Food allergy. Vomiting, spitting out the food, diarrhea, hives, rash, worsening temperament, difficulty sleeping. Consider seeing an allergist for testing or simply switching formulas to a "hydrolyzed" protein formula.....But those taste awful and most children hate them.
Varies. The classic picture of protein allergy is colitis; abdominal pain, bloating, failure to grow properly, and blood in the stool. There can be skin symptoms (eczema), respiratory sx (asthma), and many others less common.