Thank you for your question regarding your dog who seems to be excessively panting. As you can probably well imagine, there are many different things can cause constant panting. Heavy panting can be caused by heart problems, lung problems, fever, or bronchitis. You can read about this here: http://www.petplace.com/dogs/panting-in-dogs/page1.aspx
As you probably will have read whilst on PetPlace, dogs pant for various reasons. They pant to cool down, but they also pant when they are under stress, in pain, or afraid. There can also be medical causes such as neurological problems, respiratory disorders, and Heart problems such as heartworm, anemia, and fever, and even sometimes a toxicity! Sometimes in older dogs, there can even be a tumor to blame!
If your dog is panting but has no other symptoms, keep a watch on her. If it still seems excessive after a day or so, or if she develops more symptoms, I'd schedule an appointment with your Vet to get her fully checked over.
In the meantime, I would like you to please keep a close eye on her mucus membranes and capillary refill time tonight as follows:
Mucus membranes - flip her lip and look at the color of her gums. They should maintain a nice salmon pink color. Get her to the emergency Vet if they appeal white or very pale pink, or if they are a dark deep red color.
Capillary Refill time - this measures blood perfusion and test this by putting your thumb on her gum to apply pressure. After you release your thumb you will see the gum blanch. Capillary refill time is the amount of time it takes (in seconds) for the gum to return to a healthy pink color from the blanced white color. If 2 seconds or less don't worry - if it is taking significantly more time, again - off to the emergency Vet.
If this is ongoing tonight and you are really worried about your girl then I really would encourage you to just take her to the emergency Vet now. It is always better to be safe than sorry in these situations and although she could just be hot, anxious or painful, there may also be something else to blame here!
Best of luck with your girl and please let me know how you get on. Also be sure to check out: Canine Health a book I have helped write on canine health problems for future reference for your girl. I hope this helps and just reply if you need more info won't you!
Dr M D Edwards