Maybe you went in a 7w6d and the sac was only measuring at 5w6d and that was the concern. Did they schedule you back in 1 week?
I was 6 weeks, went for ultrasound (transvag) and didn't see a fetal pole. only a yolk sac. I was worried because 2 previous pregnancies I had the same thing and miscarried.
so they repeated in a week (at 7 weeks) and saw a fetal pole with a heartbeat. measured everything, and told me i was now 6 weeks, that i must have only been 5 weeks last time, and it didn't show up yet. (i really was 7 weeks, measuring 6 - because at 13 weeks i had caught up, and was measuring 13 weeks again instead of 12)
I went in a 5 weeks and 1 day and all they saw was a sac. You probably o'ed later then you thought. Congrats to you!