Constantly late period/strange times of day?

I started my period when I was 16, and up until I started college, it was always around the 13th of every month. When I moved in with a college roommate, my period changed to her schedule (started around the 4/5th of every month). After a semester, I moved into a sorority and my period changed to around the 24/25th of every month. After a semester there, I got married and my period has changed monthly, sometimes being in the first week, other times the last week, and this month I was a month and two weeks late, and I just got my period today. (Last month it was on the 6th, now its on the 22nd. Also, sometimes my period changes from starting in the morning to starting in the evening. This also fluctuates. With a fulltime job, marriage, and other work, I have a stress level that I keep as maintained as possible. Also, the pregnancy tests have kept on reading negative, and I've been to the doctor regularily each month, and no signs of kids in me. The doctor keeps saying its probably stress from work, but it's an unchanging level of stress. Could it be something else? Please help.