Doctor said 11 weeks, Ultrasound says 6

Hi All! :) I found out I was pregnant on 2/20, and by the first date of my last period (1/14) the doctor told me I should be 11 weeks right now. I had my first ultrasound on Friday and it read the baby was at 6 weeks. I have a little bit of a long cycle (avg. Is 31-34 days) and ovulate later than the usual 14 days, and think I was ovulating the weekend of the 5-6 or a little after, and had implantation the week after (I started getting symptoms the week of 2/14).

The doctor had mentioned the baby could have stopped growing at 6 weeks and that and I need to come back this Friday for a new ultrasound. She had me so scared saying it could have "short circuited" and just stopped :( she wasn't sure if she saw a heartbeat, she said she "thinks she saw a little flicker". I have a sac and little bean and everything come up on the screen. The reason I'm confused is by their math, 6 weeks was at the end of February. I took my blood tests on 3/11 and everything came back perfect with my HCG/quants. Wouldn't my levels have dropped by then if the baby stopped growing? I have been a nervous wreck and wanted to reach out and see if anyone has encountered this same thing :( I have had no bleeding or cramping. My boobs hurt less, but still are tender when I press on them, and I have bouts of nausea still and am still feel tired. My symptoms have decreased, but are still there. Praying that I just conceived later than they think .. if anyone has had the same thing happen I would love to hear more. Thanks for your time!!