According to WebMD, the presence of brown blood during menstruation is very common and unlikely to be cause for concern. Shades of dark brown or even black can also be expected. The reason for this color change is due to the body not releasing older blood quickly enough.
Additionally, as WebMD states, heavier-than-normal bleeding is not cause for seeking medical attention unless that bleeding persists over months. If it does, complications such as anemia can arise. It is best to seek medical attention if abnormal clotting continues because it may indicate miscarriage, hormonal imbalances or uterine fibroids (leiomyomas), a type of noncancerous tumor found in the uterus.