How did you get your due date? Did you have an ultrasound? If so, how early in the pregnancy was it?
Why do you think you could have gotten pregnant before your period?
Also, do you know all medical counts of pregnancy (the number like "6 weeks 3 days" or "14 weeks 3 days") begin on the first day of the last period you had before getting pregnant, or the computed first day of a last period if your periods are irregular? If a doctor says to a woman, "Congratulations, you are 8 weeks pregnant!" he means 8 weeks since her last period began, or about 6 weeks since conception. The count is done this way because the period is a big, obvious signal, not because the doctor thinks you are pregnant when you are on day 1 of bleeding. That's why pregnancy, which takes 38 weeks, is counted as being 40 weeks long. They add a spare two weeks at the front to count back to the presumed first day of the last period.