measuring behind first ultrasound

Allison • Mommy to Jonah 2/8/18🐳🌈
So I took a pregnancy test before my period bc I felt sick at the time I thought I was 4 weeks so I scheduled my first ultrasound for when I thought I would be 7 weeks but when we had the ultrasound I only measure 5 weeks which means I got a positive pregnancy test at only 6dpo is that even possible? And is it normal to measure early at your first ultrasound