Lack of a menstrual cycle (your period) is often a sign of malnourishment. At the age of fifteen, if you are not having your period, your bones will not fully develop and will become more brittle. It is not safe for you to starve yourself; this is anorexia. If you are trying to lose weight by starving, you need to get help. Talk to someone, a family member, a teacher, or a counselor. Starving causes WAY more health problems than just weight loss.