Dates off?

My first OB appt is on Monday but I could not wait to see the baby and I went to a private ultrasound clinic today. They did an abdominal scan and all they were able to see was the gestational sac and told me I was approximately 5 weeks. I thought I was 7 weeks (according to my last period). Has this ever happened to anyone? Could it be that an intravaginal ultrasound will be able to see better? Can’t wait til Monday!