Your dog may have experienced a seizure even though there was no drooling or shaking. Seizures can manifest in many different ways. You can read about seizures and the different kinds here:
A dog may also "faint" in which they fall to the ground usually on their side, stare straight ahead and seem paralyzed which indicates a heart problem.. This is called syncopy and can be read about here:
I would have your dog seen by your vet as soon as possible and limit your dog's activity until she can be seen in case it is heart related and not seizure related. If you have a video camera, and this happens again, taping the behavior will help your vet determine the cause.
Given your dog's age, either cause is a possibility. Seizures can occur for a variety of reasons including low blood sugar (hypoglycemia) from not eating to liver dysfunction or even seizures of no known origin. I'd definitely have a senior checkup done on her including bloodwork.
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