“Why does night time urination set off stomach bloating, cramping, and reflux symptoms?”

I am a 63 year old male. I take pantaprozole for a hiatal hernia, rousavastin for cholesterol, and tamsulosin for excessive urination. I also take Miralax to help pass material. Every night about 2 or 3 am I get up and dispense a large volume of urine. Within seconds, my stomach is filled with gas, bloating, sometimes cramping, sometimes nausea. I experience belching and nasty flatulence for the rest of the evening, which usually carries over to the rest of the day. I don't seem to have normal stools. Either small flimsy-looking light-colored stools or explosive diarrhea. I have a constant dull pain in my lower abdomen. I have had 2 abdominal scans, an upper GI, a colonoscopy, a bladder fill test, and urinary tests, all negative. A colorectal specialist believes I may have non-relaxing Pubo Rectalis Syndrome, and increased my insoluble fiber intake to 20 grams a day, in addition to therapy to teach me how to relax the rectal muscle, but has no answer for why my gas symptoms are initiated by nighttime urination. Despite the fiber and bowl exercises, I remain feeling miserable.