You mean distention, right? I have the same problem. The area below my naval looks like it has an inflated bike tyre inner tube inside. It was terrible at Christmas before starting the diet. My trunk looked inflated from my stomach to the bottom of my abdomen.
One cause is unbalanced gut flora. Bad bacteria create a lot of gas when you eat prebiotic foods. Some people with IBS follow a FODMAP diet, which is basically a no prebiotic diet. It can relieve symptoms, but clearly it’s not a solution to anything. It might be worth keeping an eye on how many prebiotics you consume though. If your diet was bad long-term prior to starting here, you just won’t have the tolerance to fibre and prebiotics, so gradually increasing them as the balance of your gut flora shifts towards the positive could be a good idea.
All yeasts produce carbon dioxide when feeding too (that’s how kefir gets its fizz), so candida could be contributing to your problems too. Poorly digested foods and allergens can cause the same kind of symptoms as well. It’s a tough one to work out, but if you can help the beneficial bacteria in your gut flourish, you should eventually find a cure.