We had a dog that had a grand mal seizure unexpectedly in the woods one day. Thankfully, we were close--he was so disoriented afterward that he wouldn't have been able to find us had we been out of sight.
The good news is that it was a one-time deal and never recurred.
Our only theory about Gauge's episode was that he seized due to low blood sugar. How long had it been since your Mia had eaten? I hope the seizure doesn't recur, but if you have a camera or a phone with video capability, if it happens again try to get a video of it for the vet to watch.
She's a real beauty, Susan! I hope she has no more troubles!!