How old is your dog?
What breed is she?
Is this a sudden onset or has it been coming on a while?
This is concerning at her age there are a couple of possibilities. Panting can be due to a fever - you can check hers by inserting a thermometer in her rectum and 100.5 to 102.5 is normal.
If it is more than panting and labored breathing, this may be due to a heart condition or a lung problem. I would get her in and let your vet listen to her heart and a chest xray may be in order. A heart problem would be the most likely thing and can show as exercise intolerance and difficulty breathing. Sometimes a cough accompanies this, sometimes not. If you think her belly looks distended, this too, can be a sign of a heart problem.
I hope this helps. If it does, please click accept. If you have more questions, let me know.