Hello and welcome to the forum. This is indeed perplexing. You did not have anything at all revealed during examination and testing? That's frustrating. Could this be an issue since oatmeal is a trigger of too much fiber? Something else to look into is carbohydrate intolerance. https://www.merckmanuals.com/professional/gastrointestinal-disorders/malabsorption-syndromes/carbohydrate-intolerance. However, that would most likely would only be the situation if you have a long standing history since childhood of gi issues. People also can develop an oat allergy as an adult. https://www.healthline.com/health/oat-allergy
As to your question if your usage of Doxycycline would result in a long standing post usage side effect, that is unlikely but not impossible. Anything is possible. In the mean time, discontinuing triggers would make sense such as no more oatmeal. I'm sorry I don't have better information for you but this is a very unique situation that I hope you get answers for.
Finally saw an allergist who thought my sensitivity to oatmeal was rare but has been seen in children. I only discovered the link by accident since oatmeal is prescribed so frequently as the go to food when you’re nauseated. She also believed that antibiotics could affect the permeability of stomach linings. ( I’m not sure I’m explaining this part accurately) She did not think my linking nausea with antibiotics as so far fetched. In any event, more than a year since the nausea and violent retching has ceased which coincidentally corresponded to my cutting out my favorite food .
Thanks for responding. I’m hoping that it’s diet related and the gastroenterologist set up an appt with an allergist. However, while my symptoms do seem related to what I’ve eaten there might be a neurological component which fit my symptoms. Morning nausea is sometimes an unusual side effect due to positional changes and pressure changes in your brain. My dinner time hunger and nausea which is always how I’ve characterized it is also right after I exercise. I don’t know if pressure related. My nausea occurred one time atypically when I was in a car driving up a mountain. So the most depressing possibility is that I have a brain tumor and the nausea is related to pressure changes in my brain.
I know entirely bizarre and after a year of looking up symptoms, the pressure changes are a sobering explanation. Im not in a rush to find out but will schedule additional tests and hope for the best. Thanks again.