I would keep an eye on your cycles for the next few and see if this was just a once off or if this is a new 'normal' for you. even women who have longer cycles can still have a 2wk lutal phase which is 2wks before af is due, so instead of Oing on cd15, it would be cd 20-21. You will also have to add on a week or two to any due date when you conceive on a longer cycle too.