Measuring 1 week behind/Ovulation date

So I had my first ultrasound yesterday and I thought I was 6 weeks 3 days by my app, but the sac only measured 5 weeks and a couple days. We only saw the sac, nothing else. My LMP started on July 28, so that would mean that I ovulated on the 11. I KNOW I didn’t ovulate on the 11 because I track my ovulation and we definitely did the “deed” on August 15, not before. I am almost positive that I ovulated on August 17. If I go by this, then I am spot on at 5 weeks and a few days no? Help!!! Lol