A nurse admits a normal vaginal delivery to the maternity unit 2 hours ago. The patient's fundus is firm at the umbilicus. On admission, her vital signs are BP 108/64, Apical 90, RR 20, and Temp. 98.6°F. Suddenly, her lochia appears to be heavy, saturating the entire chux pad within 5 minutes. At this time, the nurse's first priority action is: