There's all kinds of things post-partum that can make your periods wonky.
Also, if you're breastfeeding, it can supress your period because it messes with your horomone levels, which means that your periods can be rare or irregular. I don't think my period was regular until my little one was about eight months old... then I got appendicitis. Which didn't interfere with my period so much as mess with my head.
Here's my first day of periods after my little was born (if it's TMI sorry
June 12 2017 baby born
Nov 28, 2017
Jan 4 2018
Feb 6
Mar 27
Apr 23
since then its varied between 23 & 28 days with a 25 day average. I am still nursing and had a copper T IUD in a 6 weeks after birth.