Suggest Medicine For Severe Cough

I am a 67 year old female. I weigh 101 pounds, am 5'1 3/4" tall. I came down 4 days ago with a bad cough, after having same 4 weeks ago. At that time I took no antibiotic and it mostly went away. However, because I had it recently, I went to the doctor and he checked me out. Said my chest was clear, gave me antibiotic to take only if my cold got worse. Yesterday my nose started to run a lot, but mostly clear. I took Claritin but that did not help. I could barely breathe because of the stuffiness. The cough has mostly gone away. My temperature with a digital is 98.4 (I usually run 97.3-6). I don't feel great and this morning my nose is better but still quite runny. Should I just take the "Z-pack" and be done with it? Would it help? How can I tell when I should start an antibiotic in this case?