What Causes Heavy Discharge During Menstruation?

i am having really heavy bleeding between periods that last up to 2 weeks then I rest for two weeks then it carries on for another week or so then stops, its been happening for about 6 months now... I am very scared, when I went to a hospital they said it was a hormonal imbalance, and I got a huge bill for an answer like that. What do you think is wrong with me?