Food - Breastfeed - baby constipation

Hi -- I wanted to understand what you all eat so that you lo doesn't get constipated..my Lo is 4 weeks and only poops once in 2 days and really gets fussy and cries a lot and it started happening when I gave her formula once in night .. Also is it possible that mothers diet also effect them. I'm breastfeeding her but sometimes in night I need to give her formula as after feeding she still feels hungry and gets cranky.