Formula & Breastmilk

My baby was exclusively breastfed up until now she is 7 months and I introduced enfamil gent lease to over a week ago. Now she has been constipated for four days now and has not gone poop. She used to poop twice a day faithfully. Now I see her struggling and trying and it won't come out and I know she is very uncomfortable. My question is, what formula have any of you moms who have transitioned or are doing both formula and breast milk used or are using with no issues??