3-weeker baby hasn't pooped for 4 days!

Christa • Pregnant with baby #1 in November 2016! Yippee!
Hi all,
We are 80% breastfeeding and 20% supplementing and baby girl hasn't pooped in 4 days!
Took her to Instacare and she's not backed up. Doc said there is barely anything even in there!
Has this happened to anyone else? 
I hope she resumes pooping soon!