Women tend to loss their menstruation if you have a large sudden weight loss over a very short time frame. The body fat levels decrease rapidly and the body does not have enough estrogen for menstruation. You did not state your age. At around 50 years old women go into peri-menopause which menstruation become irregular; and then eventually into menopause which your menstruation stops. If the above does not pertain to you, then suggest going to a Gynecologist to be checked.