If I have regular cycles, is it possible that I may have had a delay in ovulation, making an estimated due date that was based on my last menstrual period (LMP) inaccurate?
Yes, it's entirely possible. This is one of the most common reasons for inaccuracy of the due date. On the other hand, if the doctor's ultrasound and LMP agree on a due date within 7 days, go with the LMP. If the ultrasound calls the due date by more than a week's discrepancy, a second ultrasound a month later can confirm this change in due date.
Of course, every baby and every pregnancy has it's own clock. You really have a "due month," not a due date, as a baby can come (and be considered term) anywhere between 37 and 42 weeks. So even if there's fine tuning of the due date, the baby may yet come on the date you moved away from. If the time you're trying to pinpoint is in only a two-week window, you're never going to know for sure.