Would you be kind enough to give me your considered opinion concerning the usage of the expression in bold in the following sentence?
But her question caught me off guard.
catch someone off guard = catch someone off one's guard; unprepared
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.