Hey there.

> Abnormal uterine bleeding or excessive bleeding (Menorrhagia) is menstrual bleeding that that occurs at regular intervals, but lasts for excessively long periods of time. Heavy menstrual bleeding occurs generally when you soak through enough sanitary protection products to require changing more than every hour or two, or when you have a period which lasts over seven days. Other symptoms of menorrhagia include spotting or bleeding between periods or during pregnancy. Menorrhagia is the most common type of abnormal uterine bleeding.

http://womenshealth.about.com/cs/abn...erinebleed.htm

> Women are often are frightened when they discover dark clumps of tissue in their menstruation. Dark brown or black clots in your menstruation are usually a part of the endometrium (uterine lining) shedding and nothing to be concerned about.

http://womenshealth.about.com/od/men...menstruati.htm

I'd mention it when you go see your doctor next. It might be possible that you have some kind of infection so it would be good for them to rule that out.