the next thing...
(the) next thing (one) knows
All of a sudden; out of nowhere; without warning. I was standing in line, minding my own business, when the next thing I know, a gunman runs into the bank and starts screaming at us to get on the ground! Tammy had only been on a few dates with Mark, but this one had been going smoothly. Next thing she knew, he got on one knee and proposed to her.
the ˈnext thing (I knew)...(informal) used when somebody tells a story and wants to say that something happened suddenly or unexpectedly: I was just walking down the road and the next thing I knew someone was pointing a gun at my face.
See also: next