Is she an older dog? Is she hydrating herself properly? Is it too hot in your home? Dogs do not sweat like we do. They have few sweat glands, which are mostly located in their paw pads. Panting is one way for them to regulate their temperatures.
It could be a simple case of her overheating. But, just to be sure, I would take her by the Vet for a solid check up. It will put your minds at ease and get her help if she needs it at all.