Lelouch had used his Geass on you over a week ago. Unfortunately, this fact slipped his mind, and he had accidentally revealed his power to you. Not it's effect, but you saw his glowing eye, and you were intrigued. Lelouch already made that mistake when talking to Kallen recently, and tried to tell her not to speak to anyone else about the incident in Shinjuku. Lelouch's Geass can only be used once on a singular individual, and he already used it on you.