I have a very similar thing going on. I can tell that pressing on my inner buttock (onto the pelvic floor muscles) increases my discomfort, so rectal pressure may well be a factor. I sit for long periods at my job. GI and I still are in the process of eliminating AS as a cause so I'm not much help to you. Just wanted you to know that you're not alone with your symptom. Hopefully we'll both know more soon.
BTW I have recently finally determined there is a connection between constipation and my low back/butt pain, for what it's worth. I think I've been chronically impacted for so long that I didn't think there was a link. But lately things have been moving better (hooray for BEETS!) and I can tell there is a link. But sometimes I swear I only feel the back pain so I think it must be that which is causing my LRQ pain. Of course the two are related since that part of the spine apparently controls the abdominal organs. No wonder the source of some pain is so hard to determine. Good luck to you!
That's interesting about the B12 neuropathy. I didn't know that could be a cause as well.
36 year old mother of a fabulous toddler.
Diagnosed with Crohn's July 2007. I believe it's in the right side of my colon.
Currently taking Entocort 6 mg/day (tapering down), Asacol, Protonix 40mg/day.
On Specific Carbohydrate Diet (SCD) since late July and seeing improvement in "output" quality and hopefully therefore absorption. Taking SCD legal supplements: B vitamins, Folonic Acid 800mg, EnZym Complete DPP IV Aid by Kirkman, and Saccaromyces Boulardii (a probiotic yeast).