If the weather is too hot then your dog can show these signs and it is just due to normal weather change. But if your dog is showing different behavior and there is too much lack of energy that is making it lethargic then check its temperature immediately. Its temperature should be between 102 degrees and 100 degree. In this case its breathing and pulse rate would not be normal. Check its eyes. Are they also showing symptoms like bulge, redness, and yellowish, dull or discharging something? Take it to vet soon as your dog is suffering from fever.