Constipated 4 week old :(

My poor baby has been pooping clay like stools ive realized its his formula im using earths best organic sensitivity i was BF & switched to that. He screamed out of nowhere when he pooped the clay like stool and he grunts and strains so much idk what formula to try and i dont wanna switch to something thats gonna make him feel worse anyone has gone through this? I need help :(!!