Hi AMT and welcome! I think that you have to take the best course of action for your situation. If it flares up really bad afterwards, then as much as you know breastmilk is the best source of nutrition for baby, it's not much good if Mum can hardly move or pick baby up because of her arthritis, so you have to take a holistic approach and look at the entire picture.

BW, if you do have to go on MTX, then you would have to have given up breastfeeding anyway - it will have adverse affects on bubs and you can't BF while taking it. But I really hope that it works well for you and you don't get any side effects from it. I imagine you would be taking a much higher dose for your arthritis than I am taking for my p.