LO’s poop

I switched to formula and with breast milk between the day as my supply is dipping. My LO is 25 days old. Usually can poop 6-8 times when drinking BM. Now only once a day and she’s really trying hard to push her poop out. Now she just pooped continuously on 3 diapers as I change her. And poop is not loose, it’s pasty. Maybe that’s why it’s difficult for her to push out. Her milk is nan pro 1. Any suggestions? Is it a concern? Should I change the milk?