A 78-year-old woman is admitted following a fall and found to have a fractured neck of femur. Her routine pre-operative blood tests show: Na+ 122 mmol/L, K+ 3.8 mmol/L, Urea 5.2 mmol/L, Creatinine 88 µmol/L. She is clinically euvolaemic and takes bendroflumethiazide 2.5mg daily for hypertension. What is the most likely cause of her hyponatraemia?