Back dated US/Miscarriage?

I had an US on Monday and they back dated my pregnancy by a week (back to 6w). I saw my doctor today and she told me that the fact that I know when I ovulated but the US is dating later might mean that I’m going to have another miscarriage. The baby isn’t growing at the expected rate. There was a heartbeat of 126 but it’s on the low end of the spectrum (120-170 expected). I have to have another US in 1 week and then see my doc again after. They did blood work to check progesterone and HcG levels. Has anyone gone through something like this? I’m feeling very scared and unsure about what’s happening with me and my pregnancy.