Measuring a bit behind?

I had my first ultrasound today and according to my LMP I should be 6 weeks 4 days. The ultrasound showed the smallest gestational sac measuring 4 weeks 6 days.. she rechecked my HCG and progesterone.

Hcg was 5,857. Progesterone 19.19. She wants me to come back for another scan on Monday.. I’m also on oral progesterone as I’ve had a previous miscarriage at 9 weeks 3 days in January. I’ve had two successful, healthy pregnancies.

Has this ever happened to anyone else? Any success stories? She said I could have possibly ovulated later than what I thought or implanted later. Or it could be a second miscarriage.