If you are carrying too much fat overall, it is likely that your bulging stomach is due to your extra weight. This fat on your stomach should be an unobtrusive continuation of the curves of your body, not a pronounced bulge on the lower abdomen. If you're not overweight yet you still have a pronounced fat stomach, don't blame the size of your belly on fat. The answer here is not to diet more and eat less, but to look at other reasons why your stomach is extra rounded, and different ways to reduce belly fat.
Poor muscle tone in the abdominal region is another reason for a fat stomach, but it's not likely to be the only cause. Your abdominal muscles need regular work to keep fit, just like your biceps. So if you have not been doing regular core-building exercises, chances are this is one of the reasons why your stomach is not flat.
If you have been doing hundreds of sit ups for weeks and not seeing any results, guess what? Sit ups are not going to give you a flat stomach, but they have probably given you a sore back. Traditional sit ups only exercise some of the muscles in your stomach, but not all of them. By following a simple regimen of core-building exercises, you can gain a strong set of muscles that will reduce belly fat and make your belly flat.
Most people don't have an ideal posture, sitting in front of a computer or TV screen is enough to end up with bad posture, and this contributes to having stomach flab. In order to correct your posture you have to have the desire to do so and then the right exercises to get your back muscles into the right shape. The weaker your stomach muscles are, the worse your posture is going to get, and it's a vicious cycle as lower back pain reduces ones desire to do abdominal exercise, and the problem just gets worse.
Depending on your diet your fat stomach could be a matter of bloating from eating the wrong types of foods. Some foods cause us to retain fluid, for women, this can become worse at certain times of the month, meaning that for 2 weeks of every month, fluid retention is a problem. If you body is having problems digesting certain foods you may be feeling bloated and possibly have a problem with gas. If you are eating too fast you could be inhaling air with your food, giving your belly that bloated appearance.