I did a quick search on this matter and this is what I found.
Here is the link.
http://www.gynalternatives.com/abnormal_uterine_bleeding.htm
Other Causes of Abnormal Uterine Bleeding
Although usually a woman will stop having menstrual periods if she is pregnant, bleeding during pregnancy is not uncommon. If the pregnancy is abnormal, such as a tubal pregnancy or impending miscarriage, the bleeding may be interpreted as being from a menstrual period rather than a complication of pregnancy. Because pregnancy is such a common cause of abnormal bleeding, a pregnancy test should be among the first tests in anyone who is sexually active, even if she is "sure" she couldn't be pregnant!
Thyroid disorders can cause heavy bleeding, as can disorders in which the blood does not clot normally. A physician can test for these problems if they are suspected of contributing to the abnormal bleeding.
