Maybe you could Google Sacro iliac joint dysfunction. I have it and many, many people are misdiagnosed with herniated disks, that many of us have with no symptoms, when it is the SI joint that is the culprit. This is why so many back surgeries fail. Google Richard DonTigny, who is the leading authority and you may recognize yourself. I worry because MRIs are not usually done in the SI area and often the diagnosis is missed?
When pain come on suddenly it can be from bending and twisting and the SI joint gets misaligned which can be extraordinarily disabling.
Also one will often find trigger points in the Quadratus Lumborum muscle, which attaches below ribs and to top of pelvis. If this muscle spasms, no pain killer works. If you press hard in the area you might find exquisitely sore spots and you can release these points. Pm me if you do and I'll tell you how, it is easy. Also people often get relief by pressing both hands into the waist area, on both sides.
If the SI gets misaligned, like two puzzle pieces jammed together wrong, the pain is horrible and it takes some skill to realign and the relief is immediate, although one remains a bit sore.