new due date? can't get the math right

Emma-Jane • 🌻🌻🌻
My lmp was around the 23rd of November last year, conception date was 14th of December last year, the doctor thought that I would be 7 weeks exactly today. With due date of 29th of August 17. 
Went for my first ultrasound today and the technician said baby is only measuring around 5weeks 1 day. New due date 11th of September 17. 
So I'm not actually 7 weeks I'm 5 weeks 1 day? Needing reassurance