A 70-year-old woman with rheumatoid arthritis is taking methotrexate 15mg once weekly and folic acid 5mg once weekly (on a different day). She develops painful oral ulcers and a sore throat. A full blood count reveals: Hb 9.8 g/dL, WBC 2.5 x10^9/L, Platelets 90 x10^9/L. What is the most likely cause of her symptoms and blood results?