My baby struggles/strains to poop

Hello,

First time mom here 👋🏼

My breastfed newborn is 3 weeks old and for the first week could poop just fine. He would make this cute little straining noise and then bam 💥, poop. NOW… for the past two weeks he screams and fusses every time he needs to pass a bowel movement. He’d be fine one moment and then the next he’d start pushing and screaming as if he was in excruciating pain, even in his sleep.

Is it something I’m eating that could make him feel constipated? My doctor tells me it’s normal but it just sucks to hear him in such pain!