Have you spoken to a gynecologist about this? There may be a physical reason this is happening to you, and maybe there's an easy solution. I'm not sure of all the things that could affect menstruation or how they affect it, but medications can affect it, as well as weight loss or gain (affects hormones), illness, STDs, stress and age.

Here's a link about Menorrhagia (excessive menstrual bleeding) that might be useful to you: http://www.uptodate.com/patients/con...uYAcTHKKUbOVEn