She clearly trusted you, perhaps was even flirting with you. Head in your lap, no qualm about being topless for a backrub...honestly dude, I'd set this night aside in your mind for a moment. Do you like this girl? Are you interested in dating her? If so, reciprocate that interest by asking her out to lunch, shopping, coffee, whatever. See where the conversation leads. Don't bring up the boob part or offer another backrub.

Both boys and girls struggle at this age, dropping hints but being scared to express their interest out loud. She could be hoping you notice and take the initiative.