You likely have leaky gut and the food is passing in the holes in your intestinal walls. You could also have high histamine levels which needs to be reduced with vitamin C.
You need to reduce the inflammation in order for leaky gut to heal and a great way to do this is to eat anti-inflammatory foods and supplements such as fermented cod liver oil.
Oats can be a common problem for some people. I would move on to buckwheat next if possible.