Abnormal uterine bleeding (AUB) is bleeding from the uterus that is longer than usual or that occurs at an irregular time. Bleeding may be heavier or lighter than usual and occur often or randomly. AUB can occur: As spotting or bleeding between your periods. After sex. The most common causes of such bleeding are uterine fibroids, uterine adenomyosis, or endometrial polyps. Fibroids are benign masses in the muscle layer of the uterus (myometrium), while adenomyosis is a condition in which the lining of the uterus (endometrium) grows into the myometrium. reproductive hormones not working normally – this is common in women approaching menopause or in women with polycystic ovary syndrome (PCOS) stress. vaginal dryness. harmless changes to the neck of the womb (cervix) – this may be called cervical ectropion or cervical erosion.