I'm glad that they checked you for fibroids, because that was my first thought. Another possibility is an endometrial polyp, but that should show up on your ultrasound as a thickening of the lining. When someone has "mini-hemorrhages" and the work up is negative, we often suspect adenomyosis. This is a form of endometriosis. The abnormal endometriosis implants form within the muscle wall of the uterus, and can make it enlarged, and irregular. Adenomyosis can also cause pain.
I have two thoughts for you:
Have you considered the Mirena IUD? Those really work, and because the progesterone is only dispersed locally within the uterus, you don't have to worry about systemic side effects ( like blood clots, etc). The other possiblity is an endometrial ablation. Since you only have two kids you may not be ready for that yet--you should be finished with your family before having an ablation--but it could really slow down the flow.
One thing you absolutely should do is take a multivitamin with iron. Anemia could make it very hard to keep up with those litte ones!