ultrasound and dates

Charlee
So, I thought I was around 8-9 weeks based on my period. Of course this was the ONE period I wasn't tracking because I kind of just stopped obsessing about getting pregnant, then boom. Pregnant. 
So I went in for an ultrasound today and I'm measuring 5 weeks 5 days. We were able to see the embryo and my husband and I, as well as the tech, think we seen a flicker but too early it seems to tell. Anyone else run into dates being way off? Wondering if I ovulated super late? I'm nervous because I had a blighted ovum last year and I just want this so bad. It's so hard to stay positive. 
I should mention I got a positive pregnancy test may 9th. Bloodwork was doubling as well last month.