It's possible that your dog's abdomen is bloated, due to a condition called 'bloat', where the stomach twists inside, but this usually causes discomfort, excess gas, and the dog won't be able to find a comfortable place for herself. If this is the problem, it must be diagnosed immediately, and treated, ASAP, by the vet.
If your dog is not spayed, she might have developed a serious uterine infection called pyometra, which also must be identified and treated, immediately, by the vet.
Intestinal parasites might cause a swollen abdomen, so a stool sample would have to be analyzed to check for those.
She may have ingested a non-food item which is causing the bloating, and if she's urinating and defecating normally, eating and drinking, and her gums are a good color, that's great, but there's no doubt that she needs to be seen by the vet, ASAP, so the cause of this problem can be identified and then, resolved.
I hope she'll be fine!