I had a similar thing with my Poodle. Kept coughing so took her into the Vet. They x-rayed her and her heart was fine. We do not have heartworms where I live so knew it was not that. They had me worm her two weeks in a row, and put her on Prednisone. She was on it once a day for a week, then every two days for a week, every three days, for a week, then every four days for a week. She was fine for a couple of weeks and then back to coughing again. Started back on the prednisone, same routine and has now been four weeks with no coughing. Never did really know the cause of it but it was a real hacking cough, then she would act like she was going to throw up but never did. Hope that was the end of it.