I have a severe pain in my chest and can't breath well what's the cause? I have RA! I am on methotrexate

See details. How would anyone expect a doctor on the internet to diagnosis the cause of chest pain without being able to examine someone? Call your rheumatologist. The possible causes are many.
See your doctor. Methotrexate may cause very serious, life-threatening side effects. Talk to your doctor about the risks of taking Methotrexate for your condition. Chest pain , shortness of breath, dry cough, fever, fatigue and excessive tiredness are all reasons to see your doctor fairly quickly while on methotrexate.