Hi nochran,
Constipation can cause bloating and thus, the appearance of a large belly. Especially if you've always carried extra fat there, the area will look even more bloated. Although I am not able to consult to you regarding IBS (you should visit your doctor if you have concern that you could be suffering from that) gas pain doesn't necessarily indicate a major problem. Fat also does not "create" gas, that is contigent on the foods you are eating, etc.
I recommend that you stick with your diet, drink plenty of water, and incorporate a steady regime of cardio and strength training. You should be aiming for at least three days of cardio exercise for 30-60 minutes each session and 2 days of total body conditioning. Regular exercise will not only help to burn fat and calories, but will also help you to feel less uncomfortable. Be sure to strengthen your abdominals by doing several sets of controlled, deliberate crunches every third or fourth day.
Keep with it and hopefully you will begin to see major improvements in your stomach area.
Good Luck,
Paul