Unfaithful?

I recently started a new job at an office (my first big girl job yay) and I've noticed I'm fantasizing quite frequently about one of my office mates. Normally this wouldn't bother me but I've been in a happy, fufilling relationship for almost a year so I figured this kind of thing would have stopped by now. Does it make me unfaithful that I'm finding myself being attracted to someone else in addition to my boyfriend?