5.5 wks empty gestational sac

Hi all. This is my fourth pregnancy. My first resulted in miscarriage at 9 wks- sac was measuring at 4 wks 6 days and there was the gestational and yolk sac- no embryo. My second was my beautiful, now 3 year old daughter. Fast forward to this January I had a chemical pregnancy . We decided to stop trying then found out I was pregnant this past Sunday. Ok calculation 5.5 wks . Went for ultrasound gestational sac was visible with a tiny spec inside. Sac measures 6 wks . What worries me is there was no yolk sac or embryo. I know 6 -7 wks is the best time to see embryo and heart beat, but I was pregnant barely 5 wks with my daughter and we saw everything. Anyone been through similar experience? Waiting for blood results for levels going back in a week for next ultrasound. Also my only symptoms at this time is swollen breast, with my daughter my hips spread, swollen breast, nausea before 5 wks. Thank you sorry for long rant