Empty gestational sac measuring 6w1d — anyone have a similar experience?

I had an elective abdominal ultrasound today and I’m looking for experiences from anyone who has been in a similar situation.
Based on my dates, I thought I would be closer to 6w4d–7w. I had sex July 22, 23 and 28, got my first positive pregnancy test on August 5, and my hCG was 218 on August 7.
At today’s ultrasound (August 28), they could see a gestational sac but couldn’t see a yolk sac or fetal pole/baby. The gestational sac measured 1.35 x 1.19 x 1.25 cm, with an average of 1.26 cm (12.6 mm), which the machine dated at 6w1d.
This was **abdominal**, not transvaginal. I have a transvaginal ultrasound with my doctor on Tuesday, so obviously I know that will give me much more information.
I’m trying to prepare myself for either outcome, but I’m curious if anyone had an abdominal ultrasound around this point that showed just a gestational sac and then had a normal follow-up transvaginal ultrasound? Especially if your dates made you think you should have been a few days further along.
Also, the ultrasound tech mentioned more than once that I should have my doctor make sure it isn’t twins, which confused me because we couldn’t even see an embryo yet. Has anyone had something similar happen?