Could she just be too hot? Cavaliers often have heavy coats, and if they are sleeping on our beds, can get too hot. Also many people have their heating up too high for their dog to handle. We turn the heating down and put another jumper on
Cheaper and better for the environment too!
Alternatively panting is also a sign of pain - if she is panting that much, then it would concern me. Sadly it's often a symptom of Syringomyelia, which is usually much worse at night. It would be worse reading though
http://sm.cavaliertalk.com/ and seeing if she exhibits any other signs.
You can print off some information to take with you to your vet - even if you do not thinik it is this, it is a good opportunity to help the vet learn more about the condition.
I'm not sure where you are in the world, but many vets still do not know anything about Syringomyelia or the symptoms.
A diagnosis is not the end of the world, many of the dogs are managed very well for many years on medication, alternatively, severe cases do occasionally have surgery.
Is she coughing at all? Some form of heart disease is also a possible.
I think the first step is a vet check to ensure that nothing is wrong.
Keep us posted, and please feel free to ask any questions. Although obviously we are not vets and not medically qualified, many of us have considerable experience of Cavaliers and can help.