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I am sorry your girl is having trouble.
Can you tell me how often this happens?
Is this most often in the middle of the night?
What time of day does she get her antacid? Is is given on an empty stomach?
Is she coughing at any other time of day?
What time does she eat her last meal for the day?
Of this form of coughing and white spittle coming up is associated with acid build up in the stomach. You want to be sure she is getting the antacid on an empty stomach as this medication works better in an acidic environment. I would give this 30 minutes before her last meal. If she is not eating this morning she may still have an upset stomach in that case a second dose of the antacid may help.
Another possibility is intestinal parasites, worms can crawl into the esophagus and cause irritation if you are not using a monthly heartworm prevention that also covers other intestinal worms it may be worth having a stool sample check by your vet. This is very inexpensive and can rule this out as a cause.
Allergies can also cause this problem.Reverse sneezing or collapsing trachea may also be what you are seeing as this is often confused as a cough.
Here is some additional information
Here are videos of what these look and sound like
I hope this has been helpful,please reply back if you have additional questions or concerns