Calculations on point....

So, had the oddest cycle last month but wont go too far into that cuz it would be a novel itself!! Anyways, i am late on af, neg tests but did a due date calculation and my estimated conception date was exactly on a day we bd and hadnt 8 days before that....id be 13 days past conception day. Odd huh??? Not that I'm expecting to be pregnant as weve been ttc 3-4 years and never had anything happen. But was shocked to see the dates line up. Could it be possible that im testing neg at 3.5weeks?