I'm afraid that only a vet can answer your questions about the cause of your cat's seizure and the likelihood that it will happen again. I have never heard of either a nightmare or a hairball causing a seizure, and I doubt if either one could.
It would be a very good idea to get your cat to a vet as soon as possible.
Laurie
It would be a very good idea to get your cat to a vet as soon as possible.
Laurie