Ultrasound confusion

CH

I had a miscarriage November 30th. I didn’t have a period after miscarriage and got pregnant right after. 3 weeks ago my doctor said I was 5 weeks and 1 day.. I went in yesterday and they got an accurate measurement of 5 weeks and 6 days and a heartbeat of 128! I was thinking I was 7 weeks and 5 days.. has this happen to anyone else? All seems to be okay tho!! And my doctor says sometimes the reading can be off... I’m just confused.