I'm confused?

Lizzy • I'm a first time mom to my handsome little boy who was born 8.26.17💙 he makes me incredibly happy. follow our journey and I'll follow yours!😘🙂
I had my last period November 29th through December 4th... I was due on January 1st took a test on the day my period was due and positive right away. Then took another at a clinic and it was positive they told me I was around 5w3ds and then today I went in for a ultrasound and they said I'm like 4 and a half weeks maybe 5. How does that work? Ovulated late?