Hi, and thanks for your medical question on eHealth forum!
Take a blood pregnancy test to measure beta HCG. If this is negative, then consult a doctor to look into other causes such as stress whether emotional or physical, hypothyroidism, polycystic disease of the ovary, pituitary tumors secreting prolactin, anemia and poor diet, hormonal imbalance, unexplained or sudden weight gain or weight loss.
Since a confirmed diagnosis cannot be given on net please consult your gynecologist regarding this. Hope this helps. Take care!