I am not sure I can be of much help, but I do share your pain. I have dealth with nausea since I was a kid. It comes and goes. Lately it has been bad. On occasion I have an "attack" where I feel extremely nauseous and have D. Sometimes it will go away pretty quickly, other times I will stay sick to my stomach for days.
I have yet to find out if nausea is a "symptom" of IBS. Actually I hear conflicting opinions. My gastro says yes, while many others look at me like I am crazy.
I have found nothing that helps me. My gastro put my on zoloft and all it has done is level out my anxiety a bit (which really is a good thing, but it is the nausea I want gone). I see my gastro at the end of the month and I am very close to begging for a new stomach =)
I hope you find some answers. Good luck.