First ultrasound

Went to my first prenatal visit this morning, I thought I was 8 weeks because my last period was July 27 and I got a positive test August 26, but the baby was only measuring 6 weeks 4 days with a strong heartbeat. Wondering if anybody has had anything similar?
My midwife did not seem concerned at all, but I just find it strange that I got a positive that early if I am only 6 weeks 4 days!