You are not describing a typical gran mal seizure, but your description is compatible with seizure behavior. Idiopathic epilepsy seizures generally start between 4-8 years of age. She is on the high side for this, but it is possible. Other causes of seizures in older dogs include low glucose levels, electrolyte imbalances, liver disease, hypothyroidism, some infectious diseases, toxins and brain tumors are a possibility. Pain is another possible cause of the signs you describe.
The first step in diagnosing the problem is a vet visit including a physical exam and some diagnostic testing. A common basic database includes a fecal, urinalysis, complete blood count, serum chemistry and thyroid hormone level. A test for Lyme. Erhlichia and Anaplasma exposure may be indicated if you are in an area where these tick borne diseases are prevalent.
Once you get a diagnosis or at least rule out some problems, a treatment plan can be formulated.
For now, keep your girl quiet. Offer her food and water. Make a vet appointment in the morning.
Let me know if you have follow up questions.