6 weeks pregnant ultrasound nothing but showed nothing but yolk sac

Hi, I'm 6 weeks pregnant today and i went to the doctor to get an ultrasound but the was nothing but a yolk sac seen is that normal? My last period on 17/04/20 and I had 2 miscarriage before so I’m kind of worried ? [23/06/20] Updated on my second ultrasound at 9 weeks My baby isn’t measuring where it should be... possible miscarriage??? No bleeding 🩸 or pain...