Had your girl been spayed? The reason I ask is she could have had pyometra (if not spayed) -an infection of the womb- which can be very dangerous, and can come on very suddenly even in a dog who is normally fit and well. There even is a condition equally dangerous which CAN hit spayed b-i-t-c-h-e-s, and that is "Stump Pyometra" (where the tissue left behind after spaying can get infected) This is not very common, but can happen.
this is how my boxer got but she was 1 1/2 yrs. old. we went to the vet regular and had been given a clean bill of health and all of the sudden her abdomen swelled up like she was pregnant. sh had filled with fluid and within 3 days time, she was dead. get her checked.
First...are you getting really hot or humid weather?
The heat here, (in UK at the moment) is really getting to a lot of dogs. The ones who normally race around all day, are not able to in the heat, and sit around panting. Best thing too, as hard exercise isn't good with the sun beating down. It's better to run at dawn or twilight.
On the other hand, it may not be that. Have you noticed any other changes? Or just the dwindling energy reserves?
Taking her to the vet to have her heart and lungs listened to, and investigated further if necessary might be a good plan, if it's nothing to do with hot weather where you live. But even so, it wouldn't do any harm to have those checks.
Also, you could get the vet to run a complete blood and urine screening. That should give a pretty comprehensive picture of her general health.
Any slight imbalances which show up, can be sorted out in the early stage -nipped in the bud-
Sometimes dogs show small changes in their normal patterns, well before actual illness strikes. And it's up to us to be vigilant (which you are obviously being.)