My dog trembles for days before seizures after eating. Is this pre-seizure?

That's a great question and one we do not have a lot of information to provide a definitive answer. It is actually uncommon for owners to report a pre-ictal phase. In the rare cases that do, typically it is head twitching lasting less than a minute prior to octal period. Now the post-ictal period can last from hours to days (up to 72 hours in my experience). In your pet's case, it may be that she has a prolonged post-ictal phase and then begins to seizure.
If your pet began having seizures after the age of 7 years, then it is less likely she has idiopathic epilepsy or liver disease (though hepatic encephalopathy must be ruled out). Patients that have late onset seizures (>7y) are most likely to have intracranial disease such as an inflammatory condition or tumor.
I would advise you to document the episodes and consult your veterinarian to discuss diagnostic tests and treatment.