Welcome, MRH321!
It's too bad that you have rheumatoid arthritis. It can cause quite a few problems. But so can the methotrexate. Have you tried omitting it for a while, just to see if the symptoms clear up? According to this, it can cause itching:
Methotrexate Side Effects: Common, Severe, Long Term - Drugs.com
If you're taking anything else--prescription drugs or OTC drugs or supplements--you might want to check into those as well if your doctor hasn't done this already. Some drugs interact with one another in odd ways too.
I hope that you can find a way to relieve your symptoms.