Going by your history it seems that you are most likely suffering from the condition which is known as Spondyloarthropathy
(IBD related). Generally people who are suffering from inflammatory bowel disease
(Crohn's) are prone to develop arthritis
generally involving the sacroiliac joints, which in your case is giving you lower back and buttock pain. Remicade
which you are getting for your crohn' disease will be of help in this as well.
DR JASNEET SINGH