If her gums are white but her blood level (PCV is okay) you need to have heart evaluated for a problem with perfusion.
Arrythmias in the heart cause a "skip" of perfusion and the dog can "faint" and be pale. There is usually minimal changes in the bloodwork. This workup requires and ECG and possible xrays. If your vet is reluctant to look into this, it may be time to change vets.
Let me know if you have further question.
Dr. Vamvakias
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Dr. D.A. Vamvakias
Veterinarian
