Constipation in newborn?

Katie

Told by health visitor it’s normal for some babies to go 3 days without poop. My daughter is 11 fats old and was constipated yesterday and pooped a lot last night after massage. Today same again but no poop this evening? She tried to push and has been passing gas but no poop. Doctor says is normal for dual fed babies. I did change her formula yesterday morning. Is this why!