Uterine bleeding almost always results from an endometrial biopsy. Since this bleeding is caused by injury, the opinion of the posek of this site, R Yehuda Henkin, is that it does not render the woman niddah. The bleeding may go on for about a week, and for that time can be attributed to the procedure. Other authorities are less comfortable permitting bleeding that comes from the uterus, even if a clear source of injury is seen. Therefore, it would be best to ask a specific psak of your rabbi, if you have one.