Ultrasound showed I’m only 6weeks...

I had my first ultrasound appointment today. I went to it thinking that I was already 8 weeks pregnant, however, when I was being told my results, she said that baby was appearing to be a lot smaller and that the heart rate was only 80, which would mean two different things. 1. I’m actually 6 weeks pregnant and my due date is going to be October 22nd. Or, 2. Possible sign of a miscarriage.

This is my first pregnancy and I am so excited to be starting a family, has anyone else experienced this before? Is this normal to have a miscalculation if you have irregular cycles?