Hello again, and thanks for your additional information.
I'm glad you've already had her evaluated by the vet, and she was found to be healthy. I'm also very happy to hear you will be having her spayed, soon!
If she's not on any monthly flea preventative, the white flakes you see might be flea eggs, if they're very small and look like grains of salt. If they're bigger and do resemble human dandruff, they sound like dry skin particles and this might be prevented by adding Omega 3 fatty oils to her diet, and bathing her with a shampoo with Oatmeal, like the ones at the site below:
If the vet did no bloodwork, this might be a good next step, which will be necessary before her spay surgery, in any case. Sometimes the coat can be affected by an underlying medical condition affecting the liver or kidneys.
Don't overbathe her, as this can dry out the skin and rob it of natural oils, contributing to the white flakes.
I hope she'll be fine, and if her symptoms worsen, please have her re-evaluated by the vet, as soon as possible.