The Vets didn't make a possible diagnosis? She probably does have mucus. I have gone to the doctor because i had mucus congestion, and they prescribed me an anti-biotic. So maybe your dog has mucus in the trachea. Maybe you should take your cocker to the vet to re-check.
A visit to the vet is definitely in order. She could have tonsillitis. People don't realize it, but dogs (and cats, too) do have tonsils and they can get infected, just like in people. She could also have tracheobronchitis (kennel cough). The only way to tell for sure is to take her to the vet.
Ghilly