If this is new then I would suspect it could be a case of kennel cough which is a virus dogs get (they often get it when in crowded situations such as a boarding kennel which is how it got the name but any sick dog can pass it along).
You can read about kennel cough here
www.marvistavet.com/html/kennel_cough.html
But as your dog is an older dog you may also want the vet to check her out as this could be from heart disease.
If she gets worse, won't eat or drink, acts lethargic then get her to a vet ASAP otherwise just make an appointment to have her checked if you think it could be heart disease rather than the virus which normally lasts only about 10 days.
Hope this helps you!
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