A 78-year-old woman falls at home and sustains a fractured neck of femur. She undergoes hip hemiarthroplasty. Post-operatively, on day 3, she becomes acutely confused, agitated, and visually hallucinates. Her observations are stable, and routine blood tests (FBC, U&Es, LFTs, CRP, Calcium) are within normal limits. Her medication list has not changed significantly. What is the most likely diagnosis?