4 days late on my period but haven't had sex?

Cici • Mama and Etsy shop owner! My etsy shop is SerenadeBaby
I'm 2.5 months post partum, I had my period last month on the 13th and now I'm 4 days late on my period. I am breastfeeding also. I know I can't be pregnant, but just wondering what's up with my period! Especially since I'm due to get my birth control tomorrow! 🙄 anyone experience kind of weird periods pp?