A 74-year-old woman has been taking Methotrexate 15mg once weekly for rheumatoid arthritis for the past year, along with Folic Acid 5mg once weekly. She presents with increasing fatigue and shortness of breath on exertion. FBC shows: Hb 105 g/L, MCV 108 fL (80-100), WCC 4.5 x10^9/L, Platelets 150 x10^9/L. What is the most likely cause of her macrocytic anaemia?