Could it be a miscarriage !?

Sit
So my first day of period was feb 29th 
I was due to ovulate from the 16th to the 22nd... I went to my first ultra sound and the doc said I should be 10 weeks but my ultra sound only shows 5-6 weeks! My doc said I could have either ovulated later than I expected around the first week of April or the baby prolly stopped growing and it can be a miscarriage :( 
I asked her if I was 5-6 weeks when would I have ovulated she said around April 2nd I looked at my calendar and I did BD on the 30th of March and 3rd of April so mayb that's when I conceived!? 
Ps I didn't get a positive test till April 24th and literally tested everyday since April 5th which was when Af was due 
What you guys think!? I'm so scared I have another ultra sound on the 25th and if they don't see a heart beat by then it's gonna be consider a miscarriage :(