I know how scary a seizing animal can be!
I just went through this with my 15.5 year old Wendell a couple of months ago. I rushed him to the vet, who kept him for observation during the day (as I had to go back to work). But so far, it was a one time incident.
I had a female foster seize on me . . . rushed her to the vet and same outcome - no seizures since (and that was 3 years ago).
As stated previously, get your fur baby to the vet for a check up as soon as possible. In the meantime, try to stay calm and keep your baby calm and safe.