Have you had a look at the range of booklets NRAS do? There is one especially that is very useful to hand in to your employers which explains all about it and how it could affect your work and what your rights are.
I know that MTX can cause mouth problems but I've not heard of an immediate reaction like yours. I would ask your rheummy if you have one.
YES I'VE CHANGED, PAIN DOES THAT TO PEOPLE.