why is fibre causing me lower gut pain?

I have pretty much mastered my diet and know what I can and can't have to avoid pains. I'd like to say I have cured my IBS with slippery elm powder, high quality lactose free probiotics, betaine HCL and avoiding a list of particulare foods. I drink plenty of water - from 2 - 3 litres a day, but if I have more than 2 teaspoons of slippery elm powder with each meal or any other form of fibre - from psyllium to beans, whole grains to too much brocolli, I get severe lower gut pain. If anyone can help explain this I would be gratefull.

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