Well, your vet would let you know if heartworm was a concern in your area. And I would use whatever he or she recomends. If there is any possibility that it is heartworm, you don't want to use a preventative now. You have to get the treatment from the vet. It probably isn't heartworm, as she is pretty young to have built up enough to cause a cough I think.
Could it be that she is sneezing, not coughing? Sometimes sneezes sound like a string of coughs.