Is my baby lactose intolerant?

Maybe. A baby can develop transient lactose intolerance after a bad diarrhea. It seldom lasts more than a week or two, and healing is delayed if lactose containing feeds continue during the illness.You really didn't offer much to go on.In the future please list the kids age and other symptoms when you ask a question.
No. No, lactose intolerance does not exist in infants. The earliest this entity will show up is after 2 years old and likely still very rare until age 5 years. No worries on this front, this is not your problem.
No. Lactose intolerance is common among adults, but is extremely rare in babies. All mammal milk (including human milk) contains lactose, and the chances of a newborn being lactose intolerance are a million-to-one. People become lactose intolerant later in life (teens, usually), almost never as babies.