catch (or caught) me off guard could mean: to be discovered doing something when you did not expect it. For example, you are engrossed in an intimate phone call to another woman when your wife suddenly walks into the room.
It could also mean the feeling you may have when you hear or read something that really surprises you.
Or it can mean to interrupt someone at an inopportune time – like to ask someone to do something suddenly when they are unprepared. I think there is an element of "aha! I caught you!" (Like the man talking to the woman when his wife comes in)