Hi Beth,
Sorry to hear about your back - just what you need when you have RA to contend with!
I have had a bad lower back since my teens but when I was first diagnosed with RA last year my consultant was quite concerned about my lower back and the possibility of Ankylosing Spondylitis so she referred my for an MRI scan. The results didn't show any RA related problems, just the dodgy discs I already knew about but it definitely joins in the chorus when my other joints are bad.
It could be that you're really altering your posture when other joints are bad and putting extra strain on your back, see if you can see a pattern such as bad back after particularly bad knees or after favouring a particular side when one arm is bad. I'd definitely recommend talking to your rheumy about it and maybe ask to see an occupational therapist if you think it's because of posture.
I haven't had any infusion drugs yet so can't comment there. I hope you get it sorted soon, back pain's just horrible
Vicky xx