I was 11 days late during my summer travels in Europe about 20 years ago.
I knew I wasn't pregnant because I hadn't been with anyone. However, being as late as I was just wasn't something I'd ever experienced before, so it definitely worried me. I'd been late before by 2 or 3 days at best, but never 11. But it turned out the best guess of why I was so late was that I was being hyper-stressed by the whole affair -- the plane trip, the jet lag, the culture shock of the differences I kept experiencing such as food and various ways of doing things, etc. Finally one night I totally broke down in tears and sobbed for half an hour in the company of two other women and told them all my woes:
-- the shower had fungus in it
-- nobody used (or gave us) napkins so I was wiping my hands on my socks at each meal
-- nobody ever rinsed off the dishes when they washed them, so the dishes were always left to dry in the dish strainer with the soap scum still on them
-- the sun never went down (I was in Northern Europe right at the June solstice and the sun barely dipped down behind the horizon)
-- I repeatedly almost got hit by cars and buses because they drove on the other side of the road so I kept stupidly looking the wrong way as I attempted to step out onto the busy streets to cross
After that very cathartic sob session, I went to bed and then I started my period the next day.
I was just way too stressed and in an alien environment and my body reacted.