Due date question ladies!

So i had my first ultrasound n my bean measured out to be 11weeks 4days(now 12weeks tomorrow). Yolk sack was gone n the placenta has completely taken over. Well my question is my last period qas July 23rd n thatd only make me 11weeks today(10weeks 3days when i had the ultrasound done) n when i calculate when i conceived using what he/she is measuring at it says my period was the 17th of July, which is obviously not true. So is my due date still April 28th n im only 11weeks? Or is it April 22nd n im 12 weeks? super confused!