Why is my dog hacking?

For the last 18 hours my dog hacks for 10 seconds about once an hour. It sounds like he has something caught in his throat. Only once has he coughed up phlegm. He is still eating and drinking. He is a little tired, but I think that is from him waking up every hour last night from the hacking. Also he was diagnosed with Kennel cough on August 18 and has been taking his antibiotics. Does anyone know what this hacking could be?