Help?! :(

Jessica
So my last period was the 1st of August which puts me at about 7 weeks 5 days. I had my first scan today and they measured baby at only 5 weeks 1 day which is a big gap but said that for 5 weeks everything looks good but I don't understand how I could only be 5 weeks I'm very scared and confused about what to think has anyone had anything similar happen???