Nausea isn't usually listed as a side effect of IBS, although a number of people on here experience it in the middle of a bad bowel session. Just random nausea, though, would, I think, be a sign of something else. Has anyone suggested acid reflux? It's a symptom of that. You might see if you can link the nausea to any certain food--that would be a good sign of acid reflux. And watch for weird stuff; for instance, orange juice gives me definite heartburn in the morning. But drunk at night, it's okay (at least it was before the old gall bladder went bad). Because you have nothing but acid on your stomach, a lot of things can bother you in the morning that don't bother you later in the day. Also, a lot of things can bother you if you go to sleep on them that don't bother you earlier.
Gall bladders also cause random nausea.