Growing out of it can be right. My son, who was only 4 lbs when he was born had that. Their little flaps (not medical term sorry) from stomach to esophagus aren't fully developed or strong yet. My grandson had the same problem as he was only 3 lbs when he was born. Just try to burp him more during the course of the bottle to get as much air out of tummy as possible. It is definitely unnerving if you aren't used to it but they do grow out of it. If doc says nothing else is medically wrong just be sure to have lots of burp cloths and wear your old clothes (lol). Good luck.