IBS is kind of a catchall for general digestive distress, so sure why not, it's IBS. I don't know that calling it IBS will help you treat it. My point is that I wouldn't get particularly attached to a diagnosis, as your symptoms could mean different things.

Seems like since you know you don't tolerate starch well you should avoid it, and see if some Betaine HCL helps the low stomach acid issue.

Fungus can grow in the intestine as well and it would certainly be interesting to observe your reaction to taking Nystatin, an antifungal that isn't absorbed by the body. If there is a reaction and you get better after a while, it was probably fungal overgrowth.

If you have no reaction to it, you might be dealing with SIBO, which can be treated more or less successfully with diet and antibiotics. But the antibiotic approach may harm your gut flora. And yes, SIBO is a very common cause of IBS, which itself is a symptom. It can be diagnosed with a H2 breath test, but I don't think the test is 100% reliable. There are more accurate (and inconvenient) tests.

I suppose it could also be a combination of both, which is all the more reason to take a dietary approach, no matter what else you do. This should also help prevent relapses.

The first steps I would take right now are reducing FODMAPs in your diet, eating low carb high fat, because carbs can feed both bacteria and fungi in the small intestine, and eating plants cooked, as they are easier to digest.

Stop with the starches. Substitute fruit.

I've been going through this exact same thing, man. I started eating fruit a few weeks ago (mostly oranges because they were on sale...but also apples and bananas...) and the difference is night and day. I digest them far better than I do starches or vegetables.

I don't really know what I plan to do after this discovery. Eat a Denise Minger style diet maybe? No idea. But the difference feels wonderful.

It sounds to me like you may have gallbladder issues. Supporting the gallbladder/liver with botanicals that support those organs, as well as making sure you're on a good diet of whole foods is key. However, you may also have a leaky gut going on causing all of these symptoms.

See my article What Is A Leaky Gut?

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