Dear An: Both Karrie and Gail have spoken correctly. What Karrie has described is treatment for prolonged/heavy periods. What Gail has referred to is the need for you to get an accurate diagnosis--that is, a reason, for your current bleeding before trying to treat it.
There are two major reasons for very prolonged/heavy periods: hormonal and what I call "structural". "Structural" means when excessive bleeding is due to actual problems within the cavity or walls of the uterus. Some examples of this would include fibroids of the uterus, endometriosis of the muscular wall of the uterus ("adenomyosis"), infections of the lining of the uterus, polyps of the uterine lining, or even uterine cancer. Most structural reasons tend to reoccur every month, and are more common in women over thirty.
Hormonal causes are usually linked to missed or erratic ovulations. As you may know, in a normal cycle, estrogen is produced all month. Estrogen is responsible for building up the lining of your uterus so you have something to shed each month. In a normal cycle, progesterone production increases following ovulation. Progesterone "stabilizes" the uterine lining in preparation for a possible implantation of a new pregnancy. If you are not pregnant that month the levels of estrogen and progesterone fall, triggering the release of the uterine lining—your period.
So, if you do not ovulate, the estrogen build up of the lining continues, but without the usual ovulation associated progesterone. Thus, the hormone levels don't decline, and the lining stays up inside the uterus—your missed/late period. Alternatively the lining can begin to shed under its own weight producing prolonged bleeding.
Causes for not ovulating are multifold: thyroid problems, pituitary problems, ovarian cysts, physical stressors (eg sudden increases in exercise, crash dieting), emotional stressors (problems with parents or boyfriends/girlfriends, finances), increased body weight, anorexia, rotating shifts at work, etc.
Given that your heavy bleed was an abrupt thing, my best GUESS is that it as a hormonal cause. Thus Karrie's suggestion about a hormonal treatment ("Provera") MIGHT be what your GYN suggests.
Bottom line, rule out an undetected pregnancy first then see a GYN or your local county family planning clinic. They can do the additional evaluation to give you the most "for sure" diagnosis and appropriate treatment to get your bleed to stop.
Yours,
Jane