Late late LATE!

Hey guys. So I am usually very regular  with my monthly cycles lasting from 29-31 days... But this month I am pushing day 41! I have done pregnancy tests and they are negative.... I am kind of at a loss! We have been TTC for the past 6 months. I have no increase in stress or anything that would cause me to be late, just wondering if anyone else has had experiences like this or has any advice to give.