Abdomenal discomfort or pain can cause panting. One of the other things I think of is a common abdomenal tumor is in the spleen and they can bleed out into the abdomen.
Take a look at the gums, or pull down the bottom eyelid and make sure they are pink.
If the gums look white or real pale you may want to get the dog to an emergency vet incase the dog is bleeding.
Good Luck,
Dr.V
Dr. D.A. Vamvakias
Veterinarian