ANSWER - My gut reaction would be that it is a genuine offer of help and protection. Of course witches do lie as much any anybody else but given that there appears to be no ulterior motives for her offering to protect you and in the absence of any suggestion that she might have anything other to gain by telling you what she did I would conclude that she had your best interests at heart. It's unusual perhaps for a stranger to have such a strong reaction to another stranger but on occasion we all get powerful vibes from people we don't know. In your case she may have seen something in you that appealed to her or perhaps she picked up something in your future that she felt she could help you with by lending you her strength and energy. It's equally possible that she was one of the New Agey "love everybody and everything" types and she had a similar conversation with everybody she encountered. You''ll never find out for certain.
What I can reassure you about though is that her actions were not synonymous with any negative magic toward you. It sounds very unlikely indeed that she wished you any harm. My conclusion would be that you were an attractive child who inspired a sense of maternal protection in a passing witch and she gave you a blessing to protect you through the years. A nice thing to do (though witch etiquette is normally not to bless unless asked to do so) and hopefully her kindness to you has been the source of support she intended.