In my book it is unusual to get pregnant on cycle day 6. CD8 is the conventional beginning of the fertile window. Your poorly protected sex with the new partner was spot on for a 28-31 cycle. Dec 31 is a good calculated date for a 3/26 LMP. If your due date is now showing earlier, it may just be a bigger than average baby.
By "u/a," do you mean an ultrasound? (The only medical use of it I can find has to do with urinalysis.)
If your due date of 12/27 was given to you by an ultrasound, when was the ultrasound done?
Did you get the date of December 31 from a doctor or just counting forward from your last period? And if so, are your menstrual months regular at 28 days or do they vary?