Normal pooping?

My girl is 6 weeks old and formula fed (will sometimes get one bottle of BM a day, but my stash is about to run out). She will normally go 4-5 days without a bowl movement and then will fill an entire diaper/blow out of it. She grunts but doesn’t seem in pain when it happens. It is not watery. She had her last poop on Sunday morning and this morning I noticed her grunting, when I changed her she had a solid poo so she’s obviously constipated. I know that switching formulas could be the issue, Sunday night we went from similac pro total comfort to the Walmart version of it. All the ingredients are the same and in the same order...could that switch really have constipated her?