The most common cause is pregnancy, but stress, a hormone imbalance, being underweight, scar tissue, and some meds can also stop periods. If you've skipped three in a row, see your doctor. Other symptoms you have will help them figure out what's going on. For example, extra hair growth, acne, and trouble controlling your weight, too, suggest polycystic ovary syndrome. And it's not unusual to be irregular when you're close to menopause.