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.