Most lower back pain is caused by inflammation in the intestines. Its usually referred to as referred pain.
But....when I was last on Pred this past July, when I finally weaned down I had the worst lower back pain ever. It felt like I had a impinged nerve or something, but got better after about 10 days. It hurt so bad when I first stood up and would ease a little as I walked. So maybe the Pred is causing the low back pain.
I don't think she is going to blame the Remicade. I do worry though that she might hike the Pred back up. Maybe you should try to get completely off the Pred and see if the pain improves within a week or so. If not, maybe a referral to a Rheumy is called for. Good luck!