I love the community aspect of HN and all, but see a doctor? I know you said tests were in vain but perhaps get a second opinion? This might not be the place to ask for medical advice.
It really is hard to say without knowing more about how you deal with doctors, but hound the doctors, don't leave until they have an explanation or until they refer you to someone who might. Getting expensive tests and having no explanation sounds like a failure on their part. Doctors are in a rush to get you out of there so you have to stick with them until you get what you want.
People--including Amish and Mennonites--fly from across America to central Indiana to visit Dr. Whitman in Brazil, Indiana. An extremely talented diagnostic allopathic herbalist and chiropractor. He'll couch some of his advice in naturopathic voodoo but it's always spot-on, from the dozen or so acquaintances who've visited him. Is it fungal infection? Overactive antiparasite/antifungal th-2 immune system? Overactive antibacterial system? Chronic wheat exposure (or dairy exposure)? Were you taking SSRIs for a long time to compound the trouble? Did you go for years at a time without truly resting night after night?
He literally looks at you, studies you, does a detailed 15-minute instrumented metabolic panel, and interprets the results to figure out how your body's excessively stimulated or understimulated.
Dr. Whitman's missing some understanding about amino acid metabolism, but otherwise he's better than I've ever seen from a medical practitioner.
I had good luck with a low carb (in my case Keto diet). Definitely check with your doc (or another doc) but you might try removing wheat and some dairy and seeing how you feel. I used to get stomach bloat and an upset stomaches but after starting Keto, I have been stomach issue freeing going on 4+ months. I'd start with a food diary and noting how you feel physically and energy wise after every meal since a doctor is going to ask for that anyways. It might help you spot some trends. The MyFirstnessPal app (free version) does a pretty good job of making this easy btw. Good luck!
I haven't gotten rid of my belly bloat, but it is vastly better. I used to look like six months pregnant. Now, I just look kind of pot bellied.
Some random thoughts that may or may not help you:
The single biggest breakthrough for me with the belly bloat was eating hot peppers combined with walking. I had habanero for a few days to do a "loading dose," then maintained my capsaicin levels with less extreme peppers. Since then, I include hot peppers in my diet pretty regularly.
I find that the biggest effect is hot peppers plus a lot of walking. When I started on doing the hot pepper thing, I added extra walking to my regimen. I already did a lot of walking because I live without a car. But, I added like a four hour walk every Saturday in addition. I have gone through periods where I have done just hours and hours of walking a day.
Capsaicin, the active ingredient in hot peppers, activates a channel in the cell membrane ("wall" makes more sense to me, but we only call it that if we are talking about plants) and it dumps fluids.
I ate a lot of watermelon. For about two years, I had watermelon something like six days a week. Watermelon is excellent kidney support.
I consumed Celtic sea salt and later moved to a coastal area and went to the beach regularly. It has been a few years since I bought Celtic because I am healthier now. But I still prefer sea salt, kosher salt or canning and pickling salt. "Table salt" is something my sons and I refer to as "industrial salt." If we buy it, it is not for consumption as food but for use as a non-toxic cleaner. We think the stuff is essentially inedible, in part because of the chemical fillers added solely for the purpose of making it more pourable.
Celtic brand sea salt is a high brine sea salt. It has more than just sodium chloride in it. It contains a variety of micronutrients. The body uses nutrients with other things to make building blocks and tools it needs to run the organic machinery we know as our body. If you are salt deficient, you are likely deficient in a bunch of different things. These things go hand in hand. Salt is important to gut function because your gut is lined with mucus. Salt is a major component of mucus.
I began keeping a food journal and got super picky about the oils I eat. My condition causes me to misprocess oils and this has a lot of consequences. I try to mostly stick to butter, coconut oil, and animal fats (like bacon -- I love bacon) (and I might be missing one on my "safe" list -- this is being written off the cuff and is not intended as comprehensive). I do my damnedest to avoid peanut oil and I try to avoid some other things. But peanut oil just messes me up big time.
For me, the trifecta for healing my gut is consuming aloe vera, coconut oil and Celtic sea salt in the same hour, whether as supplements or as part of my diet. Please note they all strongly promote diarrhea. You should start a food journal and try one new thing each week or two to get some idea of what it is doing to you. Absolutely do not start with taking some large dose of coconut oil (like a tablespoon) plus this other stuff as the first thing you try. You will regret it and you may be crapping your pants over it.
Now that I have recently moved back to a coastal area, I have hopes that I might eventually get rid of my belly bloat. It is already better and I have only been here 6 or 7 weeks.
/"flake" that no one believes. Do with that info what you will.
I have a genetic disorder. I have done a lot of research. I have a food blog where I talk about what I eat and why. It doesn't update on a regular schedule. I can point you to it if you want (or you can check my profile and probably readily find it).
It really is hard to say without knowing more about how you deal with doctors, but hound the doctors, don't leave until they have an explanation or until they refer you to someone who might. Getting expensive tests and having no explanation sounds like a failure on their part. Doctors are in a rush to get you out of there so you have to stick with them until you get what you want.