Sudden vaginal bleeding is not menstruation. Menstruation refers to the regularity of the cycle, and it is normal to have menstruation within a certain period of time. Sudden onset of vaginal bleeding can be caused by the following clinical reasons: 1) endocrine disorders, 2) taking certain medications that cause vaginal bleeding, such as contraceptives, 3) diseases, such as pelvic inflammatory disease, endometrial inflammation, or uterine fibroids, which can lead to the sudden onset of vaginal bleeding. There are also cervical lesions, such as polyps of the cervix, inflammation of the cervix or other lesions of the cervix, which may also lead to vaginal bleeding, so it is important to go to the hospital as soon as possible for examination, to understand the cause of the disease and then give treatment as soon as possible.