Periods should usually follow a fairly regular cycle, with normal cycles ranging from 21-35 days (from first day of one period to the first day of the next) and the average length of bleeding being 3-7 days. Irregular bleeding can either be random bleeding with no recognisable cycle at all or may be unscheduled bleeding between normal periods, also know as inter menstrual bleeding.
Causes can be due to problems with the womb itself or may be secondary to other problems such as an over / under active thyroid gland. Problems within the womb include polyps arising from the womb lining (endometrial polyps), polyps containing a fibroid (sub serous fibroid) or an actual abnormality of the womb lining itself such as hyperplasia or rarely cancer. Abnormalities of the cervix, neck of the womb, can sometimes cause irregular bleeding although usually these present with bleeding after intercourse.
Investigations required depend on the nature of the bleeding and other factors such as your age. Most ladies require an ultrasound scan and often a hysteroscopy (camera to look inside the womb). At hysteroscopy, if an abnormality is identified such as a polyp, it can usually be removed at the time by operative hysteroscopy.