Sudden stomach swollen within few hours, no pain. what could be reasons?

Acute belly bloating: There are lots of reasons for acute abdominal distension/bloating, & not all of them are due to the GI tract. Even "functional" syndromes may be present without underlying "disease." However SBO (paralyzed gut) and/or ascites are just some of the serious conditions possible here that warrant you check with your doctor for examination now.

Just one time?: There are many causes for bloating and air swallowing from anxiety and eating too fast are just some of the causes. Others have found reflux, certain grains or vegetables to be the cause for bloating as well. If this is just a one time occurrence, no further evaluation is needed. Otherwise, consider consulting a GI specialist. Coated probiotic may be beneficial for some.