Narcissism is the denial of oneself for the sake of others. If you buy a Ferrari to impress people you probably are a narcissist. You should only buy a Ferrari because you like Ferrari's not because everyone will like you more for having one. If you take a quiz to find out how narcissistic you are and then post the results online you probably are a narcissist. Why does it matter how other people think of you? That is the difference between the narcissist and the truly confident. One cares about his appearance in the eyes of others, one does not.
Alex the reason why she thought you were narcissist was because she thought you were trying to prove yourself to her or show off. A common theme in Shakespeare's plays is when one character condemns another he is often just condemning himself. She and her friend are probably narcissistic and try to prove themselves to others. It would be different however if you really were trying to show off to them, but if you were just conversing without the intent of showing off then they are more than likely just reflecting themselves onto you. (Remember this when women say "all men think about is sex!" because it is all they think about.)
I've also noticed this when trying to have some sort of intellectual discussion with women. They seem to think that instead of actually having a conversation with them you are trying to prove how smart you are to them. Women care more about how they appear to others than how they actually are.
This sums it up nicely---->
http://www.youtube.com/watch?v=afoBIMQzENE&feature=results_main&playnext=1&list=PLF6B94155B71F347B