A 35-year-old woman is 6 hours post-op following elective abdominal hysterectomy under general anesthesia. She is connected to intravenous patient-controlled analgesia (PCA) with morphine (1 mg bolus, 10-minute lockout interval). She reports inadequate pain relief despite pressing the demand button 8 times in the last hour, with only 2 successful deliveries. Her vital signs are stable (BP 128/82, HR 92, RR 16, SpO₂ 98% on room air), and she is alert and oriented. What is the most appropriate next step in management?