ultrasound

They thought I was 8w5d but I only measured at 6w6d. Herd the heartbeat. They said maybe the date was wrong that I gave them originally. But how can they go by the first day of your last period? Obviously I wasn't pregnant if I was on my period. Doc said either the days wrong or my pregnancy is "lagging" what does that even mean?? Another scan on the 24th which should put me at 8w2d... really hoping it shows growth. Hearing the heartbeat is a good sign though right?? My husband thinks everything's fine. It I'm just nervous 😩