I don't know if you are still struggling, or still keeping up on this thread, but I'm posting here anyways... just in case!
I had the same thing as what you are describing, only it started when I was 20 and it didn't get that severe to where it was constant! I had the digestive/gastrointestinal symptoms every day; nothing in the morning, mild in the afternoon, and severe in the evenings. All tests came back normal, and I went through several doctors, leaving all of them scratching their heads. I don't know if we have the same causes, or if the same things that helped me will help you- but on the off chance it does, I'm going to share it!
I went from doctor to doctor for about 3 years, with no results. In and out of the ER with suden strange symptoms, including irregular heart beats that made me dizzy, difficulty breathing, breathlessness, severe abdoninal pain, and sharp chest pains. Even the prescriptions wore off or didn't work in the first place! In my opinion the distended abdomen was the worst symptom, and there was absolutely no treatment for it! It was so painful and made me feel like you said, like there was gas trapped and I needed to "pop" it or something! The symptoms became gradually more severe, and new symptoms constantly piled on! Finally, I went to a natural health doctor- the first one to suggest an autoimmune problem and a change in diet. I was tested for several things, and the results said that I was severely gluten-intolerant (but not celiac), and moderately intolerant to dairy, soy, eggs, nightshades, and various other grains. I learned, through my doc and tons of research, that over time, constant consumption of these foods had done some serious damage to my digestive system. That damage lead to something called "leaky gut syndrome", which was racking up the sensitivites and making it impossible for my intestinal tract to repair itself, leading to an ever-worsening condition. None of this showed up on any scans or in exams, mind you!
So, I went all-out in fixing my diet; no more processed foods, no more processed sugars and carbs (they feed bad bacteria), no more wheat. I did an elimination diet for all of the other foods, by cutting them completely for 3 full months and then reintroducing them one at a time- I still eat them sparingly. I ended up cutting soy and grains permanently- technically I am on the "paleo" diet, a grain free and sugar free diet. After only one month of this, my distended abdomen was half the size, my other symptoms were seriously lower, and by 3 months all I had left was occasional gas and bloating around my period, and some heart burn when I ate too much acidic food. The symptoms came back when I tried eating grains again, even gluten free, so I just cut them completely.
For the occasional mild diestive issues, I do take a few supplements, which I took in higher doses when I was first starting out; Cayenne and Ginger are absolutley the most helpful for the heartburn and indiestion, as well as any stomach upset that I get (usually when I have a cold); Licorice Tea was so helpful and soothing during the early healing phase, and still helps to drink every now and then; Digestive Enzymes were essential in the beginning so that I could actually digest my food, digest it quickly, and absorb nutrients from it, and now I still take them with large meals; and last but not least, I took broken-cell wall Chlorella, which has helped to heal the intestinal walls and carry out any toxins that might further irritate the intestines.
As for diet; I find that I am at my best when I get my fiber from fruits and veggies, and some nuts and seeds, but I have to be careful with flax seeds and too much fiber! I don't eat potatoes more than twice a month, and only in small quantities- the same goes for cooked tomatoes nd sauce. My meals consist of a serving of protien, with 2 servings of veggies, and no carb-only foods or grains. I switched to raw milk for the amazing probiotics and nutrients, and this has actually allieviated my lactose-intolerance while still allowing milk in my diet. Fruits, nuts, seeds, dairy, and fats are either snacks or accompany a meal. And watch the citrus intake! No alcohol, no coffee, no smoking. I know this all sounds crazy- when I first did this I though I would starve, and didn't know what to eat! But I am sooooo absolutely glad that I did it, and now it's easy and I consider it a worthy sacrifice!
I don't know if this will help you, or what you have tried in your diet, but for me I was at a point where ANYTHING was worth a try!!! If you have any questions, feel free to ask! Best wishes, and I hope you find the solution no matter what it is! :)