Before I joined this site (about 9 years ago, I think), I never knew so many women thought having a regular period was normal. None of my friends, except the ones taking the pill, would be concerned about a day or two here or there. I lived in a college dorm for 4 years. Girls talk! No one I knew was that regular by nature.
I think a huge part of the problem is that you are analyzing the situation this carefully, and it's causing your body a great deal of stress.
When we were trying to conceive our second child, I took a pregnancy test when I thought I was probably a couple of weeks late. Negative. That seemed strange to us, as I sure felt the same way I did when I was pregnant with our oldest. I was feeling nauseous and had horrible heart burn and even threw up at the gas station on our road trip. Still, the test was negative. But the test I took the following week was positive!
The u/s showed that our son was probably conceived about 2 weeks later than the date of my last pregnancy indicated. The doctor said it's very possible that I was feeling stressed and ovulated later that month. It happens.
I think the best thing you can do is to stop over analyzing everything. Your body is not 100% percent predictable. If something feels off and it causes you concern, call your doctor. But it has been my experience that simply having a late period is generally not cause for concern.