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Your dog is quite elderly and as a result, there might be many possible causes for seizure activity including organ dysfunction and tumor. Many times the underlying cause for seizures is never found. During her seizures, the best thing is to have her in a quiet area. After the seizure is over, you can comfort her. This stage can last a few minute or hours.
When seizures are infrequent, treatment is usually not recommended. However, once a dog is having frequent seizures your vet may use a variety of treatments including a combination of steroids such as Prednisone and an anti-seizure medication such as Phenobarbital. Alternative to Phenobarbital might be Zonisamide, Keppra, or Potassium Bromide.
Here is a good link on seizures in dogs:
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, digital camera or phone with video capability, and this happens again, taping the behavior will help your vet determine the cause.
I hope this information is helpful to you.