See: catch off guard - Idioms - by the Free Dictionary, Thesaurus and Encyclopedia.
to catch a person at a time of carelessness.
Tom caught Ann off guard and frightened her.
She caught me off my guard, and my hesitation told her I was lying.
to surprise someone by doing or saying something they were not expecting
One of the larger airlines caught its rivals off guard yesterday by suddenly announcing a cut in fares.
When they asked me to babysit, I was taken off guard and found myself agreeing to it.