I'm sorry to hear that your pup has developed a cough and is lethargic a week after being vaccinated for kennel cough, and I understand that you are concerned about her.
A cough simply means she is experiencing airway irritation.
A cough can be related to heart disease, including heartworm (not possible being this young), lung disease secondary to a viral, bacterial, fungal or parasitic infection or tracheal irritation due to a bacterial, viral or fungal infection.
In a young dog infectious disease is by far the most common cause. Of those kennel cough or infectious tracheobronchitis is one of the most common causes. Because we know that she was vaccinated 7 days ago, and could have been exposed to the illness at a veterinary clinic, kennel cough would be high on my list of possible problems. Some dogs, especially those given the intranasal vaccine do get a mild case, although the symptoms generally show up within 48 hours. In her case I would be inclined to believe that she was exposed to the disease and got sick in spite of the vaccine.
I understand that she has been vaccinated but the vaccine takes a couple weeks to hit its maximum effectiveness, we cannot vaccinate against all of the agents that cause kennel cough, and some dogs will still get sick even with the vaccine. The good news is that dogs that are vaccinated usually don't get as sick as dogs that haven't been vaccinated because the immune system is primed by the vaccine.
As long as she is still able to eat and drinking well, isn't too lethargic, isn't running a fever (rectal temperature greater than 103F), and she doesn't have a green or yellow nasal or eye discharge then you can try taking a watch and see approach and using an over the counter cough suppressant if she is coughing so much she cannot sleep.
Make sure she gets plenty of fluids, lots of rest as the more active she is then more she will cough, and eats well. Add warm water or low salt chicken broth to her food. The better hydrated she is the better she will feel and the thinner her respiratory secretions will be.
You can take her into the bathroom with you when you take a hot bath or shower so she can breathe the steam which is soothing to her airways and thins mucous, making it easier for her to breathe. You can also use a humidifier in the room she sleeps in,
Her cough should slowly resolve over the next 2 to 3 weeks. Most of the time these dogs feel pretty well so it's a matter of keeping them quiet and suppressing the cough if we can.
If the cough is very bad you can use human cough/cold medications with Guaifenesin (an expectorant) and Dextromethorphan (a cough suppressant) only such as Benylin or Robitussin DM. Give 1 teaspoon per 10 to 12 kilos of weight every 6 to 8 hours.
If she is running a fever this is almost certainly a contagious infection and if she is running a fever and is lethargic then an examination is definitely warranted as pneumonia may have developed.
Best of luck with your girl, please let me know if you have any further questions.