The number one cause of a skipped period is pregnancy. Regardless of whether you've had sex, if you skipped a period, get a pregnancy test. If you're on oral contraceptives and have not missed a pill, the missed period is probably related to your dosage and can be remedied by adjusting the dosage.
Another common cause of irregular or skipped periods is physical and emotional stress. One scenario is worrying that you might be pregnant and then actually missing your period because you're worried. Other stress-related situations revolve around career changes, job loss, a death in the family, going off to college, moving, exams, and stressful workloads. It's not really understood why stress can cause you to miss a cycle, but it is considered a protective mechanism, a sort of prehistoric parachute in the female body. The body senses the stress levels and somehow decides to stop ovulation for that month to prevent a "stressed" pregnancy.

Overdieting and overexercising can also affect your cycle. For example, sudden weight loss could cause you to miss your period or cause a long bout of irregular cycles. Overexercising can also cause you to miss your period. It's not unusual for female athletes to stop menstruating when they're in training.

Another cause of irregular cycles could be a thyroid disorder of some sort. The thyroid gland regulates your metabolism by secreting thyroid hormone. When the gland is overactive and secretes too much thyroid hormone, known as hyperthyroidism, or is underactive and doesn't secrete enough hormone, known as hypothyroidism, this can interfere with your ovulation cycle. Thyroid problems occur in about one in twenty women and can be easily remedied. Once your thyroid problem is treated, your periods will simply return to normal.

Irregular cycles may be normal for your age. For example, it often takes young women several years before they establish a regular menstrual cycle, and some young women are put on oral contraceptives to regulate their periods. Women beyond forty can begin menopause at any time, and irregular cycles may be a sign of perimenopause. It's always a good idea to have a full pelvic and physical exam when you're experiencing irregular periods. There are a number of gynecological problems that can cause irregular bleeding, You'll need to rule out "organic" causes, such as pregnancy, as well as hormonal causes and hypothalmic causes, in which things like stress and exercise interfere with your ovulation cycle.