Irregular or abnormal vaginal bleeding can be caused by various underlying conditions and should always be discussed with a doctor, as it can be a sign of serious illness. Bleeding between periods or after sexual intercourse may be a sign of an STI (sexually transmitted infection) such as Chlamydia. It can also be due to a variety of conditions affecting the vagina, cervix or uterus.  Women using certain forms of contraception may experience unscheduled bleeding as a side effect. Unexpected bleeding may occur during pregnancy. Bleeding after menopause is abnormal, and warrants prompt medical attention. All women who experience unexpected vaginal bleeding should see their doctor as soon as possible for examination and appropriate investigation.