First ultrasound - thought I was 8 weeks but measuring 6 weeks 2 days??

Hey guys! I'm new to the reddit seen and I need some advice!

I went in for my first ultrasound yesterday. I could have swore I was 8 weeks (1st day of my last period was 11/10). The doctor said the baby is measuring 6 weeks and 2 days and they could not see a heartbeat. The doctor looked pretty worried and kept asking if I was having any bleeding/spotting (which I haven't had any). She told me to come back in a week for another U/S to measure and make sure baby is growing and there is a heartbeat. She said she wanted to wait until next week to do any blood work.

Should I be super worried? Someone ease my mind!!