My poor baby.

I think my baby is constipated. She's four months old. She is formula fed. She pooped early on the 18th but it was dark. She's having a hard time, I think. She grunts and grunts and barely anything comes out. What can I do? I've tried massaging her tummy and bringing her knees to her chest. She woke up at 4am screaming which she never does!