Pls help worried mama here!

I know100% my dates are right as soon as I got my AF I would right away put on the calendar which I did 12/4 now I had my scan on the 30/5 to my dates should have been bang on 7 weeks baby measured 6.6 weeks and had a nice heartbeat. Had ms on and off so I called my midwife n I asked to come in so have been to see baby and got heartbeat n was 7.6 weeks so (8weeks tomorrow) I'm unsure why I'm a week behind my dates? Anyone this happen to and all ok? My ob wasn't in so we had his offsider do scan he Said heartbeat was good. But I did mc late last year I'm just concerned why my dates r off pls pls someone comment n help 🌈