I have the exact same problem and symptoms and my cheap solution to completely and radically change diets fixed my IBS and lifelong mood disturbances in a week. It's so good that this weekend I had a "bad food" binge and I've spent 5 hours on the toilet and the whole weekend in a sorry state of mind.

I can't wait for the day until GPs will try and diagnose digestive and psychological issues with dietary changes that are not just "eat whole grains, plenty greens and less meat" which completely miss the mark.