Yes, an MRI. You may have a disc protrusion or other pathology at L5 S1 and perhaps L5 L4. pain in the buttocks , down your leg, that wraps around is L5 S1. Many physicians blame this on the SI joints, and others believe the SI joint is not responsible for such symptoms. I don't think you can intelligently get any kind of treatment, even PT, until you know what your lower lumbar anatomy is like.
Get an MRI and go from there!!
Everyone is an individual case, but I think the abdominal pain could be your soas/illiacus muscle overworking to compensate for the back spasms. My advice - find a great physical therapist who can work through your problems...because so much is involved. Get to a physiatrist, get MRI's and find out what is happening so you can address the problems.