Milk allergy

What were the symptoms your baby experienced when you realized they have a dairy allergy. I am currently breast feeding and supplementing with formula. My baby is always gassy and seems in pain until she passes gas or poops. She is 7 weeks. The pediatrician said it should just be dysfunction which they grow out of when they learn to poop but I just want to make sure I'm asking the right questions. My babies poop has mucus sometimes however not every poop. Any help would be appreciated.