Thank you for the query.
If all performed tests are negative most likely you do suffer from some kind of IBS
. The gas in the intestines is produced by bacterias and can be a result of severe bacterial infection. That is why I suggest you to try Rifaximine 400 mg 3x1 for 7 days treatment (prescribed by your doctor). This antibiotic eliminates patologic bacterias and can give many positive responses.
You should also increase fiber rich products and mineral water intake, avoid fatty foods, and have more physical activity.
Hope this will help.