I don't know about IBS, but I know the husband of a lady I worked with was denied SS disability, despite having terminal prostate cancer, a heart condition that required continous oxygen and strong medications, and he was on a narcotic pain killer AND confined to a scooter.
They could have appealed the decision, especially after he got worse, but his company just gave him leave with full pay so they didn't have to bother with jumping through all the hoops again. I guess SS thought he'd only be disabled once he was dead.
Moral of the story is you have to really be able to demonstrate that you can't work at all, in any capacity, at any job for which you have any training or qualifications and then you have to stay on them after they deny you. It doesn't matter if you can't get a job working part time or from home; if they think you can do that, then you had better do it. That, or you could delcare yourself an alcoholic like our ex-neighbor. Apparently being a lazy alcoholic bum is a disability but heart conditions along with cancer is not.