Worried about my ultrasound...

I went in for my u/s today, doctors had estimated me to be 7 weeks based on my LMP. There was a sac but nothing in it :/ the practitioner told me not to worry bc it could just be earlier than they thought- she said it measured at 5 weeks, but I happen to know exactly when I ovulated and it was 4 weeks 3 days ago. I feel like I shouldn't be worried bc the fetus is technically only around 4 weeks, so it would be normal not to see anything just yet, right? I am freaking out bc so many people have different stories and I'm just trying to figure out the actual developement of the fetus from fertilization, not LMP. Can anyone offer any experience or advice?