It's hard to say. The diet was vegan, but it was a poorly implemented vegan diet. I was basically eating a lot of fruit and legumes, far too much fruit I should say. Once I started having the digestive issues, I started reading about it on the Internet, and I came across some sites talking about copper overload, how vegan diets inevitably suffer from an inbalance between zinc and copper, but I'm not sure if it's pseudoscience or not. I do have low ceruloplasmin, which is the molecule that carries copper around the blood, so that is at least interesting, but yeah, could be coincidence. In any case, I haven't managed to fix it unfortunately, even after switching to an ordinary diet.

Your microbiome takes time to adapt to what you're eating and if you drastically increased fiber suddenly, it was likely not prepared for it. It could also have had a weak ability to deal with certain types of fiber. Often it's oligosaccharides (beans). The FODMAP process helps you figure out which fiber your gut may be sensitive to and then you can start low and gradually increase it over time.

The Baliza app was helpful in figuring out my own food intolerances.