If the doctor told you the due date of November 24 based on some other data, then you need to get an ultrasound. It's early
days yet in your pregnancy
, and this would be the right time to get one for the purposes of determining conception.
Did you sleep with anyone in late February or early March? I'm not getting, from ANY of the dates in the story including the ones you calculated from the first day of your last period, that the guy in January before your last period is a problem.