I had this occur for two years in a row for a couple weeks each. It was mostly just rolling over in bed and having to stand very slowly and cautiously in the morning.
It seemed to happen when there was warm weather in July. But after those two occurrences it never recurred.
Very disturbing and puzzling.
I got it in July as well, it too was very scary, but I would check my pulse during an episode and it was always steady and strong. Did a quick google search and as my vertigo only happened when I would change my head position I assumed it was a benign condition.