Yes it was a seizure. But do not panic. My Gunnar is 11 and has been having these since he was about 3. He has one about 6 months they last about 15 min, when it's over he rests for a while and then he is his old self again. My vet can't give an explanation but since it is not hurting him and is not often we just deal with it when it happens. I stay close to him, talk softly and just be ready to clean up any pee or poop that might happen. If you have small children keep them back as I have noticed our boy feels very insecure around the kids because he can't move. It's best to check it out with your vet and I'm sure it won't be serious but do update us.