This might be an ugly case of Irritable Bowel Syndrome, I guess. You might try soluble fiber (e.g. Metamucil, Konsyl, Benefiber, Citrucel) for a few weeks to see if it helps. Or a different antibiotic, in case the SIBO breath test was incorrect.
Also, some people get gas from Metamucil, probiotics, or diet (e.g. beans, broccoli).