so as far as you're concerned food intake or exercise was not any different the night before the unexpected low. So I believe you've had what's call 'diabetes'.......some days, doesn't matter what you do, the result will be unexpected! May be the wind was in a slightly different direction, or a butterfly in the amazon basin flapped its wings just a bit too hard..........!
Just thinking, no other lows in the past 48hrs? you didn't say what you were pre-meal of that morning?
I was wondering what that fluffy thing was in your avatar, now I know!