You may be one of those young women whose periods may be affected by changes in body weight or diet, exercise, illness, or stress. Remember that if you are having sexual intercourse and not using a reliable method of birth control, an irregular period could be a sign of pregnancy. Your period may last 1 day or it may last 7 or 8. All of your cycles may not be the same number of days, and the length of your cycle may change over time. It is common for a young women just starting her period to have irregular periods for a year or two. If you are sexually active, it is a good idea to have a pregnancy test if your period is late or if the flow is much lighter than usual.