If your dog had an unusual amount of exercise, she could certainly be very tired still, particularly if she is over a certain age. It's hard to tire young, athletic breeds out, but many dogs do start to show their age after 5 or 6. Dogs are like people in that their muscles adjust to a regular exercise routine, but greatly increasing exercise suddenly can have an impact. Best to exercise regularly, rather than seldom but excessively. If her appetite is normal, her gums pink, and temperature normal she is probably just tired. However, as hot as it is, if she had access to any ponds, which can be contaminated with dangerous algae in hot weather, you should call your veterinarian.
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