I'm sorry i didnt see this thread sooner, or i would have replied.
I was told I had "irritable bowel" and my doctor was completely wrong. I had a gallstone. I suffered for many years, time enough for it to grow into the size of a walnut, before i finally had surgery to remove it. If you think it's more than IBS, it really might be. See other doctors, get other opinions.
Yes you can be disabled by plain'old IBS. But your other problems, like your back issue, may be what really wins your case.
What your doctor notes in your records, the tests that you have, plus your doctors essay "narrative report" that he writes to the SSA is what wins your case. What your doctors say is the most important thing.
There is a list of 100% guaranteed-to-get-benefits diseases, but they are serious. Think end-stage-renal-failure. Alzheimer s.
Stuff like IBS has to have serious evidence for your case to be won.
I'd suggest lawyering up because winning it by yourself likely won't happen. I helped a relative through this process recently. SSA sent me paperwork to fill out, and I did that, supporting her claim as a family member who knows her, and knows she can't work. Her attorneys requested to inspect all forms before forwarding them to the SSA, and I'm glad they did. They threw out what i thought was a well-supported document that i had written. They said family/friend testimonies (which SSA requests) are there to help trip-up your case. You're not a doctor, so your opinion doesn't matter. But they'd sure use it to try and compare it to your own application and trip you up! Find inconsistencies! It won't help, but *could* hurt.

-See doctors regularly, and file immediately if you can't work.
-You have to pick a date that you "became disabled". All doctor appointments *after* that date can be included as evidence, and this means medical records.
-It also means all medical records *prior* to that date *will not* be considered as evidence.
-Once you have stopped working, the credits you have earned towards SS will begin to atrophy and eventually expire. It's like unemployment benefits. You can't claim unemployment years after you got fired. It has to be done immediately. My dad got screwed because he became self-employed
after he stopped working. He then couldn't even run his own business after a while...by the time he claimed SS he was out of credits and just disqualified. Like that. It's over.
-You will help your case by not working. If you are working, even part-time after your "date i became disabled" date, you'll pretty much ruin or really hurt your chances of winning.
I'm not a lawyer, but look into legal help and verify what i'm saying so you don't hurt your case.
Get as many medical tests done as you can that would show you have a problem.