Can I ask a few questions?
What was the first day of your last period?
What days after your period did you have sex?
The man you used a condom with, was he inside of you at all without a condom? (not just touching you)
it is possible, for some women, to get pregnant right after their period. Every womans cycle is different. Normally, ovulation occurs about 14 days after the first day of your last period.
The doctor will always based how far along you are by the first day of your last period. He might change your due date if he does an ultrasound, though. The ultrasound can be quite accurate as to how far along you are.