2 doctors & an ultrasound all different due dates

When I first found out I was pregnant I went to my normal doctor who said my due date will be 1st of September, then went to a different doctor yesterday & she said the 31st of august. I then went to get an ultrasound and the person that did my ultrasound said my due date is 11 September. First two doctors says I was 8 weeks & today at the ulstrasound he said I was 6 weeks 4 days.