A 80-year-old woman with moderate Alzheimer's disease on Donepezil 10 mg daily is seen by the community mental health team. Her carer reports she has become incontinent of urine over the past 4 weeks without any urinary symptoms or UTI. She is also increasingly drowsy. Her medication was recently changed - Solifenacin 5 mg was started for urinary urgency by another clinician. What is the MOST likely explanation for her worsening symptoms?