This sounds like Tracheal problems such as irritation or a collapse.
Tracheal collapse is a condition in which the trachea partially collapses as a dog breathes, leading to tracheal irritation, coughing and sometimes secondary changes in the lungs or upper airways. It is most common in toy breed dogs but can occur in larger breeds at times. It is reported to be most common in Terriers, poodles, Chihuahuas, pugs and Pomeranian have been reported to be the breeds most commonly affected by this condition but it is reasonably common in all the toy breeds and several other smaller dog breeds.
The symptoms of tracheal collapse are coughing, difficulty breathing and tiring easily. When this condition first occurs many dog owners truly believe that there must be something caught in their dog's airway due to the severity of the cough. A collapse can cause coughing and gagging for prolonged periods of time. Your girl may appear to be trying to vomit with nothing coming up. If the tracheal collapse is occurring inside the chest the increase in pressure on the circulation can lead to heart enlargement or contribute to heart failure.
Another serious condition that could be causing this is a blockage this is where a object has caused a obstruction or lodged in the digestive track. The symptoms of this is gagging or trying to vomit the object up. Change in stools or no stools at all. Loss of appetite, distended belly, panting.
At this point I would strongly recommend taking her in for a examination to determine the cause and begin treatment.
I hope she is feeling better soon, please let me know how she is doing.