Usually they pant when they're are hot as a result of exercise or weather conditions.
If this is not the case then it will be because:
1) Your dog is in pain (check for any injuries or arthritic flareups
2) Has a respiratory infection like pneumonia
3) Has a chronic illness such as diabetes or cushion's disease.
If your dog is panting heavily, but will not lay down, then it probably isn't exhaustion, or pain. Sitting up makes it easier for them to breathe
when they are not getting enough oxygen. Check that your dog's gums are pink. If they are grey or bluish then she isn't getting enough oxygen. For dogs with blue tongues and gums (like chows) you will have to check a pink area like the inside of her bottom eyelid.
**Please don't hesitate to take your animal to the vet. She may need blood work or an X-Ray for a diagnosis, or maybe just her temp taken to determine a round of antibiotics is needed. Take care of your dog, and your dog will take care of you!