What Causes Sudden Unconsciousness ?

Yesterday my partner told me she felt faint.
She collapsed and when I tried to lay her down her entire body was rigid, as if all of her muscles had tensed at once. Her pupils dilated and rolled backwards. She did not respond to my voice or touch and did not appear to be breathing. This lasted about 30 seconds before she came round with no recolection of what had happened
The same thing happened about 18 months ago and a doctor told her not to worry unless it happened again.She is also approx. 10 weeks pregnant