Repeat ultrasound Thursday

Hey girls, I'm getting really nervous for my next ultrasound, last one was a empty sac I was supposed to be 7 weeks 4 days she said to come back in 7-10 days. My periods come every 34 36 days so every 5 weeks, I do know I only conceived 4 weeks ago. It probably doesn't make sense but, my last period was the 5th of October and lasted 5 days I ovulated on cycle day 21 so does anyone get me where I'm coming from I guess my periods could be irregular? So it would only make me almost 5 weeks? I'm just want to be confident