If you haven't been diagnosed with Crohn's, how can you go ahead and suppose that you either have Crohn's arthritis or AS?
AS is far more painful than Crohn's arthritis in the spine (not to say that it isn't painful itself). AS patients have considerable trouble with moving, sitting in the same position for extended periods of time, standing, etc. Crohn's arthritis isn't as debilitating in regard to mobility in comparison to AS or RA.
If you truthfully have Crohn's, spinal arthritis or back pain is very common. Often during my flares, I complain more about my flank pain than I do about the pain in my abdomen.