No to the phantom pregnancy. Usually the bitch will take bedding, toys, rugs into the basket and guard them. Pyometra - when lining remains behind causing womb infection, is the danger in bitches.
Is she overweight? That can cause a longer recovery period in hot weather, in that they pant to cool down a lot more. Does she seek the cool tiles ? Is this a similar symptom to your other dog?
Could be painful joints so it's easier to lay down. Does she eat laying down, our friend's pooch does.