Hi Sir or Madam,
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
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.
If your dog is panting but has no other symptoms, keep a watch on her. . However, from your post, it sounds like she is in pain which might be from a disc issue or some other discomfort. Given her age, I'd plan on having her seen tomorrow morning if she has no other symptoms. You can read about disc issues here:
Now remember that she can get bloat which can cause panting and pain, but you usually see other symptoms such as attempts to vomit unsucessfully. I'll give you a site to read just to be sure this isn't an issue.
I hope this infiormation is helpful to you.