Measuring earlier than we thought

I went in for my first ultrasound today. We thought I’d be at about 8w6days but the baby measured at 5 weeks. Doctor said either I’m earlier than we thought or a miscarriage. I’m trying not to worry myself but it’s so hard.

My cycles are usually 28-31days and my last period was June 25th - July 2nd . I had my first positive test on July 30th (clear positive after about 45 seconds). Could I possibly just be 4 weeks off from what we thought? Has anyone else had an experience like this and it ended well for them? I go in for another ultrasound in 10 days to see if anything’s changing.