I suspect the panting was likely a pain issue. This pain can be from an extremity like you had checked or it could have been internally like stomach cramps, pancreatitis, gas pains, and in such a large girl the back is a high likelihood for pain as well. If it was hot then overheating will also be an issue in a heavier girl. I do suggest monitoring her for the day and if the panting continues or she continues to whine then a trip to the vet would be recommended just to be sure everything is okay. Bloodwork and possible spinal xrays will be needed.
If she is over the age of 6 years then something to consider here as well is Cushing's disease. Other symptoms you may notice is weight gain, eating more, anxiety (can be seen with panting and pacing), increased thirst and increased urination. IF these symptoms or some of these symptoms are present then I do suggest talking to your vet about testing for Cushing's disease.
Please let me know if further assistance is needed.
Theresa
A Voice for Your Pet
15 years experience in small animal medicine