There can be different reasons why a teen doesn’t get her period. The most common reasons are weight change (especially weight loss), bulimia (vomiting to control weight), over exercising, stress, a hormone imbalance, or pregnancy. For example; polycystic ovary syndrome (PCOS) is a hormone imbalance that can cause irregular periods, unwanted hair growth, and acne. If you haven’t had a period in 3-4 months, you should make an appointment with your health care provider. If you’re sexually active and miss a period, take a pregnancy test.