Pentasa and back pain

That sounds like sciatica. The sciatic nerve is frequently irritated by muscle spasms in the lower back and gluteus. Pentasa is unlikely to be the cause of this.

People with inflammatory bowel disease can have arthritic changes in their spine. More commonly, many adults (esp. women) have sacroiliac joint problems that cause sciatic nerve pain. There are easy stretches and exercises you can do to alleviate this.

Don't take any ibuprofen or naproxen to treat this. Try some ice packs on your lower back, and some easy stretches (look up piriformis syndrome for info).

all of the NSAIDs are to be avoided by anyone with an inflammatory bowel disease. They are known triggers for flares.