Pregnancy dating scan help

Hey ladies I'm needing some help, I found out I was pregnant 3.5 weeks ago with a decent positive, I went by my last period and I was due the 3rd but went for my dating scan and I'm a whole week behind, I'm confused as with my other two by LMP I was smack on but this one's difrerent and that would mean I got my positive only a week after ovulation which I didn't think was possible either??