So confused :(

I don’t understand if I’m 3 weeks that would mean I got pregnant when I wasn’t ovulating? I don’t understand this I thought I’d be like a week or 2 at most because the days of ovulation. The first day of my last period was July 26th & the nurse who scheduled my 8 week ultrasound appt said I was 3 weeks based off that my last period. Can someone who understands this stuff better than me please explain:( we’ve been tracking my ovulation for a few months trying to conceive.