This could indicate a few conditions. You could have been injured in the stomach, eaten food containing bacteria causing abnormal conditions in the abdomen, or you could be extremely hungry and nauseous because your blood sugar is low. The stomach can ache in extreme hunger.
Any number of things, but the ones which come to mind:
If you're female you could be pregnant, or near your menstral. If not, you could also be having the flu, a Urinary Tract Infection, Appendicitis, and a lot of others. Hit up the doctor if it continues.
If you have a fever, you might have the swine flu and you should seek a doctor. If the entire belly is hurting, it could quite possibly be food poisoning. If it lasts for a day or less, you could have a food intolerance such as lactose intolerance.
It could be one of a number of things, including, but not limited to, motion sickness, indigestion, too much alcohol, or even food poisoning. If you also have a fever and muscle aches, it could be flu. Try drinking a small amount of ginger ale or ginger tea to settle your stomach. If that doesn't help, see a doctor.
It all depends on the area of where the pain is coming from. If there is pain on the left that can be appendicitis, pain on the right side can be gas bubbles. If pain is generally in the middle of the stomach then your bladder or tummy hurts, meaning that you either have to poop or throw up.