Consult your gynecologist It can be due to - Pregnancy ( Which you already ruled out) Stress (mental / physical) Sudden loss of weight Eating disorders/ anorexia Excessive weight gain/ obesity Drugs (antipsychotic/antidepressants) Birth control pills PCOS (polycystic ovarian syndrom) Chronic diseases- diabetes, celiac disease Thyroid disorders (overactive/ underactive) Pelvic infection Repeat pregnancy test, if it comes out to be negative consult your doctor for withdrawal bleeding and get yourself investigated to find out the cause. But if it is due to stress or obesity or other psychological factors then you need to take action by yourself. De-stress yourself , eat healthy, moderate physical exercise, yoga. Weight reduction in case of obesity