If you google 'hypoallergenic foods' you'll find lists and other info. Rice starch is an example of something that probably can't trigger an allergenic response. Water is definitely non-allergenic. If either of those triggers your symptoms, then you can pretty much rule out allergies. I don't know if it's possible to be allergic to saliva or gastric juices that would be released in response to swallowing.
I don't understand the digestive system well enough to think of the possibilities for what might be causing the symptoms. That's why I suggest a gastroenterologist.
@Pink I share a lot of your symptoms although I don’t have a gallbladder and have to take pain medication. My understanding of the gut is that many neurological functions happen there. ME is neurological so it makes sense that gut problems are serious and complicated. I have gastroparesis and leaky gut which is slowly improving as is my digestion. A lot of things that helped me had to be added so so slowly. I can’t take hcl for instance as my gut lining is probably too thin but for now a little lemon juice is helpful. I had to do nothing but bone broth for awhile and this leaky gut supplement. I slowly added light foods. I have had to evaluate every supplement and medication as well. I discovered like you everything hurt initially.
I also had your pregnancy experience, thirty years ago and I still struggle with getting my hormones in balance but without hurting my gut. For a small percentage like me hormones are hard on the stomach but extremely necessary so I have an expensive cream I have to get compounded. I recently had a very difficult experience with molvantik. I got incredibly sick, close to calling 911. Weeks later my gut still feels raw from the experience. The weird thing is I had really good energy for awhile after, like something bad got strip mined out of my gut but what? I wish I knew what happened because I really had better energy for about a week. My guess is some nasty bacteria, virus, or parasite was temporarily knocked out.
I know that at the very least stomach pain is emotionally and physically exhausting and sets off a domino of symptoms. Usually I can’t focus even enough to write this much. I feel for you Pink. Breathing is sometimes all I can do to get through one minute at a time.