A 75-year-old woman is admitted to the hospital with an acute exacerbation of COPD. She is treated with nebulised bronchodilators, oral corticosteroids, and antibiotics. On day 3 of admission, she develops new-onset confusion. Her observations are: HR 110 bpm, BP 100/60 mmHg, RR 24/min, SpO2 90% on 2L oxygen, Temp 37.8°C. Her arterial blood gas shows pH 7.30, PaCO2 7.5 kPa, PaO2 8.0 kPa, HCO3 30 mmol/L. What is the most likely cause of her confusion?