question re. back pain and fm and ra

I am new here and I have 2 questions.

1.  I have herniated L4 L5 with neuropathy both feet and for the past 2 months sicatica down the rt. leg. One of the things my pain doctor keeps talking about is inflamation at SI joint which ra pts. get. I am not understanding that as I thought that ra doesn't affect the back.

2. My second question is I don't understand the relationship of ra to fm as I was told I also have fm.  The rheum. and pain doctor have tried to explain it to me but I am just not understanding it.

Thanks for any clarity you can give me.

Caffey