A 65-year-old woman with rheumatoid arthritis, treated with methotrexate 15mg weekly and folic acid 5mg weekly (taken on a different day), presents with painful ulcers in her mouth and nausea. Blood tests show: Hb 105 g/L, WBC 2.5 x 10^9/L, Neutrophils 0.8 x 10^9/L, Platelets 110 x 10^9/L. What is the most likely cause of her symptoms and blood results?