If you buy yoghurt, get only the plain product (no fruit or sugar) that contains live organisms. This is the treatment that is used all over the Middle East. It is natural, good tasting, healthy for you, and it works.
HI and thanks for using the forum.
Since you have already been to a doctor and been evaluated, then I can assume that there is nothing physically going on with you.
Many people have had success with reducing gas with daily use of pro-biotics. Probiotics are available in health food stores and also now in such products as yogurt. They are believed to help restore the "natural flora" in the intestines and therefore improve colon health. You could also try an over the counter gas relief medication (Gas-X, Beano, etc.) These can help reduce gas as well.
Finally, have you been ruled out for reflux? Typically, reflux symptoms include belching, however, some people do experience flatulence with this.
Hope this helps a little. Good luck.
Bridget