Mrs Jones is a 70 year old lady who presents with a 4 month history of progressive shortness of breath, non-productive cough and fatigue. She has a history of recurrent urinary tract infections and Diabetes. She is currently taking Metformin and she has had multiple courses of Nitrofurantoin prophylaxis in the last 5 years. On examination, she has bilateral fine inspiratory crepitations.
What is her most likely diagnosis?