What Causes Excess Bleeding During Periods?

i have excess menstruation .i mean it wont stop untill i take some hormonal pills...my periods were very regular untill2005 june..after that non stop bleeding...sometimes i chance 5 pads pr day..ultra sound taken in 2005 showd pco..after that i under went sonography for atleast 3 times...every thing was normal..doctors say hormonal imbalance...diagnosis has not been made...right now iam on ovral...i dont hve hair on my hand and face..not even obese..