Could be stress. Stress has a way of throwing your system out of whack, resulting in all sorts of strange symptoms, including your period going wonky. Also, it depends on your age. Many women, when they are a few years from perimenopause, start experiencing some variations with their periods. Mine started going wonky when I was in my mid-30's. Even so, go see a doctor. There is a possibility that something else might be up as well.