Anyone have brainpower to help me understand?

The first day of my lmp was August 14th, (28d cycle) 
I tested September 15th, and got my bfp
I went for my first ultrasound on October 8th which my apps were saying I was 8 weeks then, right? 
When she measured the gestational sac, she said I was way too early for a heartbeat and nowhere near 8w. More like 5ish weeks?? 
Okay. So when would I have ovulated?! Or conceived?