Anyone measure small while pregnant?

Rachel ☘️💙March’19 & 🎃💗Oct’20

So I took an early pregnancy test Feb 17 and it was a faint positive and my LMP was Jan 11 so my due date was 10/17/20. My early ultrasound measured me 6 days smaller so they changed my due date? To 10/23/20. I’ve been measuring small with fundal height and today they decided to order me an ultrasound.

I’m just worried and concerned as to why I’ve been constantly measuring smaller with later due date when I was pretty spot on (3 days early) with my March baby.