Based on the symptoms you've described, the most likely cause of your dog's current condition could be gastroesophageal reflux (acid reflux) leading to esophagitis (inflammation of the esophagus). Here's why this is a strong possibility:

Analysis of Symptoms:
Sudden Onset During Sleep:
Gastroesophageal reflux can occur when a dog is lying down, especially during sleep. The horizontal position allows stomach acids to more easily flow back into the esophagus.
Coughing and Gulping Sounds:
Acid reflux irritates the lining of the esophagus and throat, which can cause coughing as the body attempts to clear the irritation.
The dog may make gulping or swallowing noises in an effort to soothe the discomfort or clear any regurgitated material.
Continuous Swallowing of Saliva:
Excessive saliva production is a common response to esophageal irritation. The dog swallows frequently to manage the increased saliva and discomfort.
Agitation and Distress:
Esophageal pain can be quite uncomfortable, leading to signs of distress, confusion, or agitation as the dog doesn't understand why it feels this way.
Previous Mild Episode:
A prior episode that resolved on its own suggests a recurring issue. Acid reflux can be intermittent and may vary in severity.
Other Considerations:
Foreign Object:
While a foreign object lodged in the throat could cause similar symptoms, it is less likely if the dog was sleeping and didn't have access to items that could be swallowed.
There would often be gagging or retching without producing anything, and possibly pawing at the mouth.
Kennel Cough or Respiratory Infection:
These usually develop more gradually and are often accompanied by other symptoms like nasal discharge, lethargy, or fever.
Nausea or Gastrointestinal Upset:
Nausea can cause drooling and swallowing, but it often leads to vomiting, loss of appetite, and less frequent coughing.
Tracheal Collapse:
More common in small breeds and typically triggered by excitement, pulling on the leash, or drinking water.
Recommended Actions:
Consult Your Veterinarian Promptly:
Esophagitis requires medical attention to prevent complications like ulceration or strictures (narrowing of the esophagus).
Your vet may perform a physical examination and possibly recommend imaging studies or an endoscopy to confirm the diagnosis.
Keep Your Dog Comfortable:
Reduce stress and keep your dog in a calm environment.
Avoid offering food or water until advised by your veterinarian, as swallowing may be painful.
Monitor for Additional Symptoms:
Note any changes such as vomiting, changes in breathing, lethargy, or worsening of symptoms, and inform your vet.
Preventive Measures for the Future:
Dietary Management:
Feeding smaller, more frequent meals can help manage acid reflux.
Avoid feeding your dog right before bedtime to reduce the chance of nighttime reflux.
Elevate Feeding Bowls:
Raising food and water dishes can help reduce the amount of air swallowed and ease esophageal function.
Conclusion:
While gastroesophageal reflux leading to esophagitis seems like a strong possibility based on the symptoms, only a veterinarian can provide an accurate diagnosis after a thorough examination. Prompt veterinary care is important to alleviate your dog's discomfort and address any underlying issues.

Note: This information is intended to provide guidance based on the symptoms you've described and should not replace professional veterinary advice.를 한국어로 번역