A 58-year-old woman with rheumatoid arthritis, well-controlled on methotrexate 15mg weekly and folic acid, attends for a routine review. Her blood tests show: Hb 105 g/L (115-165), MCV 102 fL (80-100), WBC 3.5 x10^9/L (4-11), Platelets 130 x10^9/L (150-400). What is the most likely cause of these findings?