What Causes Abdominal Pain And Fullness In Stomach With Small Amounts Of Food?

Question: I have acid reflux, and symptoms for five years have been mostly in the throat: acid-filled mouth and throat, throat pain. Suddenly, during a meal four days ago, I could not complete the meal, felt too full, then later had symptoms of gastroenteritis -- mainly abdominal pain and fullness with small amount of food. The pain has diminished, but the fullness and general weakness continue. Also a vague nausea.
Incidentally, the increased dosage of PPI did not seem to affect the throat symptoms. Also, I mentioned that the pain had diminished; in fact, the constant fullness itself is a kind of pain. There is, too, conspicuous belching after eating anything.
---sincere thanks for any comment.

---Five or six years ago, when the GERD was first diagnosed, there was an upper endoscopy which verified it. A year or so later, there was a colonoscopy with the finding of one polyp.
---There has been no abdominal ultrasound and no culture of feces to exclude candida infection.
---Was advised by Internal Med physician to double the dosage of Omeprazole to 40mg twice daily. Will begin this tomorrow morning. (Should mention that when the diagnosis was made, several PPIs were attempted; at that time, only the Omeprazole seemed to work, if only to some degree.) To this day, the acid in the throat is a constant problem. It is hoped, apparently, that the more aggressive dosing of the PPI will help.
---Other meds, besides the PPI, include Tamsulosin, Metoprolol, as well as OTC antacids and vitamin supplements (E, B-6, C, Co-Q10, Omega-3 fish oil).
---Should also mention that, though an acid stomach has always been there (managed for years with Pepcid AC), this dramatic situation began about six years ago, and it began with similar food-poison / stomach flu kind of symptoms resulting from a processed “instant breakfast.” The attack was immediate. With this new attack of five days ago, there was the initial sense that the GERD and the gastroenteritis were separate. Now, one wonders about that and if something very serious could be the cause.
---Will certainly make diet and life-style changes. Thanks for stressing it. Also, will pursue the probiotics and the simethicone.
---Sincere thanks. Yours has been the most thoughtful response we have ever had from an on-line med-help.

---Another brief update before pursuing the tests: This morning, the diarrhea and weakness continued, but this time there was red blood in the stool. Will contact urologist who ordered a colonoscopy four years ago; at that time, there was one polyp. He mentioned that another scope should be done in five years, or earlier if there were blood.
---Again thanks for your thoughts, XXXXXXX