Lab has a cough that sounds like the heartworm cough but tested negitive for heartorms last month?

a friend of ours has a lab that was just at the vet last month for rabies shot and heartworm testing and the test came back negitive. Now the dog is coughing and it sounds like the cough a dog gets when they have heartworms. the dog has been on heartwoem prevetitive since last month so I dont really think it is heart worms could the dog have a hairball or just a cold as the weather has been pretty screwy around here for the last few weeks in the 60's and 70's during the day and down to the 20's and 30's at night (that's indiana for you lol) I am hoping that its just a cold or a hairball and not heartworms as the dog comes and plays with my 4 dogs.