Lelouch doesn't think that his Geass has many restrictions. He used it on you to ask you if you were the Glasglow pilot from Shinjuku, and tried to use it a second time, only for it to fail. This both alarms him and confuses him, as he assumed that it should have worked. This is when he discovers that his Geass can only be used once on any given individual. He must quickly think of a way to hide the truth about his identity as the one who assisted you in Shinjuku yesterday.