Measuring behind

My husband and I went to our first appointment last week. I thought I was at least 7 weeks and some days and according to my last period (Oct 19th), I should have been around 8 weeks. We did the scan and I was only measuring 5 weeks 3 days or 5 weeks and 6 days. They are wanting me to come back in two weeks (Dec 31st). Has anyone gone through this and everything turned out to be okay?

I keep testing and my tests continue to darken but I know that isn't really an indication of a successful pregnancy. I'm just so scared and overwhelmed.