It's quite common for babies on formula to poo less then breastfed babies. Maybe your LO is having a growth spurt? Wonder week? Teething? DD would randomly have clingy periods then show me something new and she had growing pains at the start of the year from a massive growth spurt (3 inches in a month...) It's really only constipation if they're having hard poos that are hurting to pass or they can't pass them

HTH