In case you are actually really slim, then your condition can also be caused by weak connective tissue or by some kind of food intolerance.
The former can be caused by hormones, malnutrition or just simple biological predisposition. You can alleviate it by doing core exercises (both front and back!) but be careful to not create too much abdominal pressure, because if your connective tissue really is weakened, you might risk an umbilical hernia. If you strengthen your abdominal muscles your belly will look more evenly distributed.
But also check if you perhaps are suffering from any kind of malnutrition or dehydration, which also brings me to the latter possible cause: It is possible, that you are consuming some kind of food, which you cannot be digested properly. In this case the food is not digested, but fermented which often results in a bloated bowel and most likely flatulence.
Here you must experiment a little bit. Check if your condition gets worse after eating certain kinds of food. Pretty common are lactose intolerance and fructose intolerance, so if you notice that after eating dairy products or fruit you belly gets extended even more, you might have found a possible cause (but don't be paranoid, any food will extend your stomach and bowel a little bit)
If you are unsure with any of this, you should also ask a doctor to rule out any actual sickness.
A general advice in case these tips won't help you: By getting toned in general you can cover up many irregularities of your body.