Care to give any context? That can mean different things based on how long you have been seeing a girl...to the way in which she said it (insecure look, excited, happy, that zoned out mode where they get a "deer into headlights" look)...to when and where...
From my personal experience...I'd say she may have had "not so nice" boyfriends in the past and wonders why you're treating her so well, she my not think she deserves it and it may confuse her a little, but she's attracted to you and as a result you can get away with it without her seeing you as a not-so-sexy nice guy...at the moment anyway.
but without context that my not hold much water.