By your 5th month, a due date based on an ultrasound could be more than a week off or possibly two, but still not by a big enough margin for error to include conception in September. If the date you're concerned about were in the first couple of weeks of August, it might be a more worrisome thing, but not September. Given that your (April 20) due date is right, it sounds like you were pregnant by the time you had sex with your ex.
At what point in your pregnancy did you first get a due date from the doctor, and by what method? (Such as, little cardboard wheel, a calendar, an ultrasound)