Hmmmm. Well, what about other foods?
(Are you having a protein source with your oatmeal as well?)
You may ultimately be having a problem with the fiber. Or having that much fiber in one sitting.
If you have oats and natural pnut butter, that is indeed a lot of fiber at once....but really shouldn't be messing with you thta much.
But if that's true, about the fiber, then i would think you'd be having this problem with other fibers like veggies, whole grain breads, sweet potatoes...etc.
If you can ingest every other kind of fiber except oatmeal...and this is a recent development, i would keep looking for a contaminant. Get a new container, try a new brand, etc.
Have you gone to a doctor about this besides your mom?