Is this a dog seizure?

That does not sound like a seizure to me. I have had dogs that have had seizures and they have all been laying down. They have a jerking like motion, sometimes for several minutes. Sometimes they will stretch their heads forward, and their legs are stiff, or sometimes they will lift one up. This sounds more like a neuroligical problem to me. Your vet needs to run some blood tests to determine the problem.