I started breastfeeding and then switched to formula also with both of my kids. My pediatrician said that it can be normal for a baby to not poop every day when they are very little. It's more about the consistency of what comes out (is it hard like marbles?) or how uncomfortable she may be (which may be a little hard to judge--they often grunt a lot when they go!). Never hurts to ask your dr. in case there is something medical that needs to be addressed.
Also, please talk to your dr. before making any formula changes. When I asked our dr. about what I thought might be constipation in my daughter as a baby, one thing he stressed was the need for iron at this age. Don't skimp on the iron unless directed by your dr. He/she can best direct you about what's most important to look for in a formula for your baby.
Best of luck with your little one!