8 week scan but only measuring 5 weeks 2 days?

I went into today for my first ultrasound. My app says 8 weeks 2 days but in the scan the lady said she could only see a yolk sac, no baby. I was measuring 5 weeks 2 days on the scan and she said I must had ovulated late only 3.5 weeks ago. I got a positive pregnancy test 4 weeks ago though??? Has anyone been through a similar situation and had a good result? I'm booked in to go back in 2 weeks and she told me to keep positive about it, the math doesn't sit with me well though. Had a chemical pregnancy last year at 5 weeks so wasn't expecting a good result today. I've checked the app and my cycle was 6 days longer than average and I did bleed for one day 12 days before the positive test but put it down to implantation bleeding.