Have your dog examined by a veterinarian immediately.  GDV (gastric dilatation and volvulus) or bloat can be a life-threatening emergency.  What happens is the stomach feels with air then twists on itself strangulating vessels.  This lead to tissue death and shock and is an emergency.
Other things that can cause a dilated stomach are tumors, internal bleeding, or fluid in the abdomen. 
Your veterinarian will do a thorough exam and then imaging--ultrasound or x-rays--will need to be done.

 The boating in dog is the problem which require immediate attention from the Vit. According to one report, the bloating is second highest killer problem of dog after cancer. The causes of stomach bloating are

1. Stress
2. Eating habits
   
3. Exercise before and after eating
4. Physical structure like old and big dogs
5. heredity.

So I advise you to visit a vit for proper diagnosis and treatment. For more details, visit Bloating in Dogs.