Well... I'm not an expert on physiology. But, the answer, I think, is that "bleeding" means something different in each of these situations. In the first, pp hemorrhage, we're talking about an open wound from which blood is coming out. The action of the uterus works to staunch that flow -- kind of like how if you put pressure on a cut, it stops bleeding. In the other, menstruation, we're talking about the sloughing of the uterine lining which is not blood but contains blood. The action of the uterus does not cause you to bleed, per se, it only helps the separation and expulsion of the lining (which contains blood) from the uterus, just like it expells the baby during birth. Uh, right?