A 69-year-old woman presents with increasing confusion, drowsiness, and constipation over the past few weeks. Blood tests reveal: Calcium 3.10 mmol/L (ref 2.20-2.60 mmol/L), Albumin 35 g/L (ref 35-50 g/L), PTH suppressed, Urea and Creatinine normal. Chest X-ray is clear. She has a history of breast cancer treated 5 years ago. What is the most likely cause of her hypercalcaemia?