What are the causes of heavy bleeding with menstrual periods.

Abnormal uterine bleeding, heavy menstruation or menorrhagia is usually the result of a hormonal imbalance in adolescents during the years following the onset of menstruation, or in women who are approaching menopause.
Menstruation is often irregular or heavy during these times because, depending on hormonal levels, the ovaries may or may not release an egg. Another common cause of abnormal uterine bleeding is fibroid tumors.
Other causes of excessive bleeding that your doctor should consider include:
* Cervical or endometrial polyps
* Lupus
* Pelvic inflammatory disease (PID)
* Cervical cancer
* Endometrial cancer.
Women using intrauterine devices (IUDs) for birth control, may also experience excessive or prolonged periods. If you experience menorrhagia while using an IUD, the IUD should be removed and replaced with an alternative birth control method.
I'm not sure how old you are, but I would recommend that you speak to your Mom if you're still under 18 about consulting with your doctor/gynaecologist for further evaluation. They will need to run some tests in order to properly diagnose you with what could be the cause of this.

It's to do with the lining of the uterus, but this can mean you could have problems like endometriosis later in life or you may even have it now and not know.
You could help your body control it more by going on the pill, as this makes your periods lighter in most cases and regular to the day.
Also, if it is making you pass out or quite ill, then your doctor may tell you that you can skip your periods using the pill so that you only have one every 6 weeks. You can do this without your doctors consent as it is quite safe but I think in your case, you should discuss this at length with your doctor.