Final answer:
The nurse should immediately assess the firmness of the patient's uterus, as the symptoms suggest postpartum hemorrhage and uterine atony might be occurring, which are critical conditions that require prompt attention.
Step-by-step explanation:
The nurse's most important action would be to assess the firmness of the patient's uterus. The patient's symptoms suggest she could be experiencing postpartum hemorrhage, which is a critical situation requiring immediate attention. A uterus that is not firm and contracted may indicate uterine atony, a common cause of postpartum hemorrhage, which can lead to significant blood loss and potentially life-threatening complications.
Encouraging the woman to breastfeed can be helpful as it can promote uterine contractions due to the release of oxytocin. However, this is not the most immediate action that should be taken given the patient's vital signs and the amount of bleeding noted. Escorting the woman to the bathroom to void and documenting the findings to reassess in 15 minutes are less critical than immediately assessing the uterine tone when such symptoms of potential hemorrhage are present.