Bleeding from Trauma (Dam Makkah)
Only uterine bleeding not due to trauma renders a woman niddah. Bleeding due to trauma is called dam makkah, blood from a wound.
Bleeding that originates outside the uterus (e.g., from the vaginal wall or the outside of the cervix) definitely does not render a woman niddah.
Contemporary halachic authorities differ in their approach to bleeding from an injury inside the uterus (e.g., from an endometrial biopsy). The posek of this site, Rav Yehuda Henkin, rules that such bleeding does not automatically render a woman niddah and much depends on the specific circumstances. Other authorities are less comfortable permitting bleeding from the uterus, even if a clear source of injury is seen. Therefore, a specific halachic question should be asked in each case.