Baby girl constipated?

I switched from exclusively breastfeeding at 5 days old to exclusively formula feeding. She's now 8 days old. First day on formula she pooped twice (I'm assuming half of it was from breastmilk), day two on formula she had one poop, and today she hasn't had one yet. Starting to get worried. Checked her temp and she is fine. Tried bicycle kicks & tummy massage. Still nothing. Anyone experience something similar? Advice?