So scared of losing this baby! Measuring a week behind

I had some spotting off and on and so my dr had me go to an outpatient imagine center and got a transvaginal ultrasound done, I’m suppose to be 7 weeks 3 days going by my last period, but the lady said she measured baby at 6 weeks 3 days, I didn’t get to hear the heartbeat but I did see the flicker, and she said it was 164 bpm and so I messaged my dr to see what she felt about the ultrasound and she said I probably just ovulated later than usual but I’m almost positive I ovulated on the 7th or 8th of December, my last period was November 23rd. I’m just so scared and don’t know what to do I’ve never had this happen before