Six months ago I had surgery on my lumbar spine at L2 - L3 and L3 - L4 to relieve stenosis (compression or pinching of the nerve roots). Although the op was only partially successful, and I still have further treatment to undergo, my 'IBS' symptoms have reduced by about 90+%. Apparently, it's common for this type of lower back problem to affect any of the organs in the pelvic area, causing some or all of the symptoms attributed to IBS. Bladder weakness or the inability to fully empty it is another common side effect.
So, if you've been given that catch-all, rubbish diagnosis of IBS and have any problems with your lower back, ask to see a specialist so you can get a scan to ascertain whether your bowel problems are, in fact, caused by your spine.
Roz