Formula and poop

Nelly

Hi mamas. Just to ask you. My little one is on mixed feeding recently. She eats more formula now then breast milk. She is popping. There is no constipation. But every two to three days there is a day when she does not poop. She trying, pushing, crying but nothing. Do you thing it can be from the formula? Do i have to change it? She is 6 week old