Newborn not pooping

Initially she was pooping fine when she drink dugro milk and my breast milk then suddenly I stopped giving my breast milk bcos I had mastitis and taking antibiotics didn’t want to give my milk and I also suddenly change her milk to similac instead but she suddenly stop pooping and having hard time to poop anyone have any advice or experience this before as I’m first time mom I’m confused with what happening rn🙏🏻