Hello, I have gone through your query and understood the concern. If you have always had regular monthly cycles, this menstrual disturbance suggests hormone imbalance since you have excluded pregnancy. This can happen with sudden weight changes, stress, infection, polycystic ovaries and other hormone imbalances and certain medications. Though Medroxyprogesterone acetate is the drug most often prescribed for such corrections, it is better taken after a proper clinical evaluation and a sonogram. Please maintain a healthy weight, regular exercise and a healthy diet. Hope this helps.