There is a concept in halakha called dam makkah, literally blood from an injury, which does not cause niddah status. Thus, for example, if a woman knows that she has a scratch on the vaginal wall that is bleeding, in most cases she can ignore blood that she sees. This is even applicable, in the opinion of many authorities, to an injury within the uterus. If we know that her bleeding comes from a cut, such as from a medical procedure, we can usually ascribe any vaginal bleeding to the injury and she will not become niddah. HOWEVER, as the rules involved are complicated, and even small variations can have major halachic implications, every situation should be individually addressed with a rabbi or yoetzet.
The doctor in your story probably based himself upon the idea of “dam makkah.” However, if the uterine bleeding is caused by natural internal factors such a separation of the placenta and not by a known injury, we would NOT apply the concept of dam makkah. This is especially true in cases such as this, where we don’t have instruments sophisticated enough to determine the exact cause of spontaneous bleeding. It is for this reason that women (and physicians) should understand the boundaries of the doctor’s expertise and the rabbi’s. We fully rely upon physicians for medical information about a given situation. However, they should refrain from drawing halachic conclusions for their patients based upon medical facts. Women should also be aware of this and should not rely upon their doctors for psak halacha.