Irregular bleeding can have a lot of causes. It sounds like this is a more common experience for you - not just a one time thing? I would make an appointment with your ob/gyn. That much bleeding is definitely problematic - not to mention annoying!
Could be a lack of the hormone progesterone.if the body doesnt produce enough of this hormone it can bleed. the more estrogen ur body has more likely to bleed and have pms symptoms and cramping, The main reason we women are lacking in prog is b/c of our diets (red meats mainly) etc etc... i would try taking progesterone pills take as directed. You could also get testing done at ur doctors to see if ur hormone levels r normal.
You may have a disorder with an hormonal unbalance it might be your Progesterone levels they might not be sufficient enough to control your menses you might want to go to the OBGYN real soon since your are menstruating twice a month the chances of you developing anemia are pretty high since you are loosing a lot of blood in one year. Also when you bleed continuously you have a disorder in which means you might be ovulating and the body is trying to throw the egg out but because you did not ovulate the bleeding wont stop because the body thinks it still needs to throw the egg out then in some cases the doctors prescribe you Birth Control to control your menses. Then again you could have a lot of things that could possibly be wrong until an OBGYN actually does test on you. Source: Personal Experience I do not ovulate on my own so I need pills to induce my menses and pills to ovulate. Good Luck! Keep me updated God bless and I hope the OBGYN helps you with your situation.