Stress, diet, environment, weight loss...It could be so many things. Then again, it could just be your body. I've had some of the most irregular periods I've ever heard of, with no explanation. I'd get it for two weeks straight, the nothing for two months and it would come for just a few days. My mom's were like that, as were her mom's. I went to the doctor and got tested for all sorts of things just to make sure it wasn't anything more serious. I suggest you do the same. In the end, I tried a few different birth control pills, and eight years later the same brand gives me regular 4-5 day periods each month.