HELP!!

My 1 month old is breastfed...mainly from the breast but with bottles if we're not home for long periods of time. Anyways, she has been having a hard time passing gas and pooping. She can do it BUT she screams and cries while pushing. And she's pushing so hard. I've tried gripe water to help, her doctor suggested prune juice...did nothing! Anything you mommies have been through and can help me with? TIA