I havent had my period in a year, im 18?

Missed periods: There could be a number of reasons. The first thing you should do is check a pregnancy test - if you have irregular periods, you could have gotten pregnant at some point this year. Excessive weight loss or exercise can also disrupt your menstrual cycle. Other possibilities include hormone abnormalities, like hypothyroidism, or polycystic ovarian syndrome. Your should be evaluated by your doctor.