It seems you have been having a quite a disturbing joint disease
. Currently you are disturbed by this sore red nodule on the back of your leg. With the kind of immunosuppresive medicines that you are taking for your RA, it is likely that despite severe infection you might not have a fever
Any red nodule which is sore could be an initial focus and should not be left undiagnosed.
I would suggest you to meet a physician for a local infection , to see of you are developing signs of infection.
Hope you query is resolved.