Dear An: Given what you have shared (normal ultrasound) it seem unlikely that the erratic bleeding is the result of fibroids or an ovarian cyst/mass. It is also reassuring that the lining of the uterus was not grossly overly thickened. It sounds like your GYN suspected anovulatory (no ovulation) bleeding as the possible cause.
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.
The Provera supplies doses of synthetic progesterone in an attempt to cause the lining to consolidate and then have an orderly shed which should stop. Since your GYN recommended that you try and resume the Provera to see if the bleeding can be controlled you should follow their instructions. If you continue to bleed let them know as they may briefly increase the dosage or have other instructions for you. It might even be that they will ask you to take a combined estrogen progestin birth control pill for several months. Alas, even a D&C is usually only a temporary fix.
When you next speak with your GYN you might want to ask about missed ovulations as a cause for erratic/prolonged bleeding. Causes for not ovulating are multifold: low thyroid problems, pituitary problems, ovarian cysts, physical stressors (eg sudden increases in exercise, crash dieting), emotional stressors (problems with boyfriends/girlfriends, finances), increased body weight, anorexia, rotating shifts at work, etc.
Yours,
Jane