Frequent diarrhea, especially watery or explosive stools after milk feeds
Bloating or a swollen belly after feeding
Excess gas or passing gas more than usual
Fussiness, crying, or seeming uncomfortable after feeding
Signs of stomach pain, such as pulling legs up or arching the back
Repeated spit-up or vomiting after milk feeds
Poor weight gain or slow growth
Diaper rash from frequent loose stools
Symptoms that start soon after consuming breast milk, formula, or dairy-based foods if already eating solids
Symptoms that improve when lactose is reduced or removed
A doctor’s evaluation, since true lactose intolerance is uncommon in infants
Medical testing if recommended by a pediatrician, such as stool tests or a feeding trial