A 38-year-old woman undergoing elective abdominal surgery receives intraoperative anesthesia with propofol, nitrous oxide, and fentanyl. At the end of the procedure, the anesthesiologist administers naloxone 0.4 mg IV to reverse opioid effects before emergence. Within 2 minutes, the patient becomes agitated, tachycardic (HR 120/min), hypertensive (BP 160/95 mmHg), and complains of severe abdominal and back pain. What is the most likely explanation for this acute change?