1 month old and constipated

My baby just turned 1 month and i feel she has been having trouble. She is formula fed but she recently moved up to 3 oz . her stools have been on the darker green side and more pasty sometimes on the more solid side. We do notice she seems to struggle alot when trying to poop and i just wish i knew what else we could do for her. I make bicycles wirh her legs massage her belly and gave her way less than 1/2oz of water. Please any mommies got suggestions???