If its a girl she maybe pregnant. They may also have colic which is when the intestines flip and or become knotted they need immediate medical attention. Or your dog may have eaten to much to fast...
If you have a young "lady", she may be pregnant. If it's a male or you know your dog can't be pregnant, it may be a blockage or tumors - I lost a dog to cancer a couple of years and the symptom was...