I was having the same issue. The doctor told me nothing was wrong but eventually I started losing weight and then looking for another doctor. I was always healthy-athletic prior to this. We found celiac disease was the cause. The immune system is attacking the gluten in wheat, barley and other similar protein encased grains. When it does so, the attack harms the intestines. If you have a liver test and it shows a slightly abnormally high AST, I think, was one of the pointers. An endoscopy was required for proper determination. After being off gluten for a few months the stomach distention was greatly reduced.
Hello. I would recommend trying to go to a Internal Medicine Doctor. I used them whenever I get something that my normal doctor can't get a handle on. Everyone I have had always got to the problem and got it fixed. Good Luck...