Is she better at night?
Sightly quiet is probably normal for the heat, but if the dog is normally a good eater I would say that going off food is not normal. If the dog is normally picky, then sure…heat will make us all blah.
Rectal temperatures are important if she is not normal as well, anything over 102.5 is high, and she will need to get out of the heat. If her body temperature reaches 104, this is emergent and she will need active cooling and a trip to the vet. Its amazing how fast the heat can catch up with dogs.
Most do fine outside with proper shade, water etc. (Except bulldogs, they never do well with heat!), but heat stroke can catch up with any breed. Best to be on th safeside and just monitor.
Regardless, if she doesnt perk up in a day or two, a trip to the vet is probably not a bad idea. There could be countless other things going on if she remains off.