What can I do to help relieve my puppy's congestion?

I just got my 2 1/2 mnth old puppy about 1 1/2 wks ago from a shelter and she was fine except for occasional sneezing. Yesterday she developed alot of congestion and she sounds like she is hacking up phlegm b/c the cough is not dry. Her activity level and eating is fine. i have taken to the vet and she is on antibiotics but i hate hearing her hacking. I want to help bring her some relief. any suggestions??? I keep seeing Robitussin mentioned but I am worried it will make hurt her