S.,
It sounds as if your daughter is having anovulatory bleeding. The simplistic definition of anovulatory bleeding means that the ovary has failed to secrete progesterone leading to menorrhagia or prolonged or excessive bleeding. Meaning this long period is not at all uncommon especially at her age.
"Anovualtory bleeding is a common and normal physiological process in the perimenarchal years of the reproductive cycle. Ovulatory menstraul cycles may not be established until a year or more after menarche. This Phenomenon is attributed to the immaturity of the hypothalamic-pituitary- gonadal axis." (ACOG American College of Obstetricians and Gynecologists, 2006 Compendium, pg. 625.
All that being said though, there are disorders that would warrant a visit to yout health care provider, such as von Willebrand's disease which is a bleeding disorder not usually diagnosed until adolescence. This can be identified by some routine screening for bleeding disorders typically blood work. Another potential problem would be anemia as you are already aware of, however our bodies are wonderful at compensating so getting her checked out to assess the severity of her potential anemia would also be a good idea.
I would get her checked out by an MD or Nurse Practitioner
I hope this is helpful,
T.