I'm not sure what kind of rash this is, but if it's all over as you describe then it's probably a reaction to something externally. If the rash develops into open pimples or blisters or wounds, they will undoubtedly get infected very quickly, so I would get her to a vet for a full assessment and treatment. Unlike humans, dogs don't tend to get heat rash (because they don't have sweat glands in the same places we do), but things like insects multiply rapidly in extreme heat, and this could be the most likely source of this rash. If the pimples are itchy, it makes the situation worse, because your dog will scratch them, which in turn will cause them to bleed - adding to the likelihood of them becoming infected.
Hi. I've just been reading up a little more about this and I think it could be the result of a Staphylococcus pathogen aggravated by hot and humid environmental conditions. It doesn't change the advice given previously, so go see your vet, as it will need treatment fairly quickly to prevent it deteriorating into open wounds.