What Causes Irregular Menstrual Bleeding?

What is the reason behind heavy blood flow over a long period of time that sometimes continues for 2 weeks or even a month in a menstrual cycle? she feels weak and her haemoglobin level is down to 6. She is 39 years old weighing 70-75kg and she sometimes have vaginal pain and the pain is so much at times that she is unable to walk. She has 3 children and all of them were delivered by surgery.