Maybe they were talking about your personality instead of your outwards appearance. I know a lot of girls that call other girls "fake" just because they of the way the girl acts. Like maybe she is two-faced or she says one thing and does another and just acts fake. I'm not saying that you're like that or anything but since you say everything about you is natural and you don't dress slutty, this is the only thing I can think of. If a girl said this to you then she is probably just jealous or something so I wouldn't worry about it if I were you. Hope I helped:)
People will say anything to make themselves feel better, even if its not true. Do YOU think you look fake? Sounds like you don't think so. So don't let it get you down! Their opinion doesn't matter if you don't agree. Don't let it hurt your confidence.