Early scan

Today I went to an early scan, I always thought I could have ovulated later the month I got caught. As I’ve always had a positive before a missed period with my previous pregnancy and didn’t get one this time till 3 days after my missed period and it was faint. I have since got my dye stealer so all seems to be going In the right direction. Anyway so I went for the scan today thinking I was 6 weeks 3 days going off the first day of my last period, there saying I’m 4 weeks 5 days, saw the sac with yolk sac & have re booked me for 2 weeks time for another scan where we hopefully see the heartbeat. Has this happened to anyone else ?