The abdominal pain
which occurs while lying down is usually due to acid reflux
. You have not mentioned the site of the pain. Usually this will be in the upper abdomen in the midline region and usually associated with burning sensation in the chest region. In severe cases patients can have cough also due to acid reflux. You have mentioned that you had got all the tests done. You have not mentioned whether gastroscopy was done or not. Because only in gastroscopy we can see the Lax hiatus and hiatus hernia
. Sometimes even the gastroscopy might be normal and still you can have pain. This is known as non ulcer dyspepsia
. You may also need to get your acid output levels.
My advice is to avoid large amounts of food intake at a time. So in a day you can have small amounts of food but many times. Avoid spicy and fat containing foods. Do not lie down immediately after having food. The head end of the bed needs to be mildly elevated. And also consult a gastroenterologist
so that he can prescribe some medication.