Babies poop worried

So my baby pediatrician switched her formula to similac neosure so she can gain weight and now she has been so constipated and poops out big turds she screams in pain every time she poops her face turns red and everything is this normal it's been happening for over a week I called her pediatrician about it already and they said she has to get used to the formula. But this is brutal. She is also only a month old and weighs 6 pounds