5 weeks or 6 ?!

Nea • First time mommy to be! 🌈 baby due 11.16.17🗝❤️
According to the hospital ultrasound on Tuesday & my LMP I'm 5 weeks. Today [friday] I went to my obgyn & she stated the ultrasound was measuring at 6 weeks & she can only see a yolk !  I'm a bit worried has this happened to anyone before ?!