Okay, this is an area where I have learned something...intermittent fasting. My question to you would be: why are you doing it? If it's to manage weight, you must also reduce your carbohydrate intake. When you fast, say at least 16 hours (which is about the minimum for any potential benefit), eating porridge/oatmeal, which is essentially 80% carbs, is going to shock you back into insulin response, which is what intermittent fasting is meant to help you BREAK!! The whole idea behind keto/low carb, is to minimize the insulin response so that you don't store unused calories in the way of fat. Fasting, and then loading on carbs, is entirely counterproductive.

I'll tell you more if you're interested, but I wonder if the discomfort you feel when you break your fast is due to the high glycemic index of what you eat at the time.