My miniature poodle did this on occasion, otherwise bright and happy but occasional belly ache and blood flecks in vomit. She got better with only one round of omeprazole. Best thing is to feed smaller amounts and more frequent meals.
If your dog is otherwise totally normal, and bloodwork is fine, its likely what you are dealng with. However if there is any change like further anorexia, lethargy, panting, diarrhea etc. then a more intense work up is probably warranted.