No heartbeat

I went to my first official baby appointment today. My doctor said I was 6 weeks according to the blood test, (8 now) and my ob said I'm 13 weeks according to my period. She did the Doppler and couldn't find a heartbeat and I have to wait 4 weeks for an ultrasound. She sounded concerned and now I'm freaking out. I think I'm 8 weeks because that's what the blood test said. has anyone else not heard a heartbeat that early?