Abnormal uterine bleeding has many causes. Thyroid and adrenal gland conditions, for example, can cause hormonal imbalances that affect menstrual periods. Fibroids, polyps, scar tissue, infection, trauma, atrophy and precancerous conditions also can cause irregular menstrual bleeding. You may also find that you are just beginning to experience the menstrual irregularities common to the years just prior to menopause, when hormone levels fluctuate unpredictably. However, your symptoms could be something more serious. Discuss your symptoms as soon as possible with your health care professional.