It sounds that she would like the thought of a marriage and family in the future and as good as the idea may sound now she may just not be ready for that nor it seems is she ready for any type of commitment as well. The best thing to do at this point is take time for yourself and give her some space and time as well and if things bring you back together in the future take it slowly and don't rush into anything. A conversation about both of your plans and goals for the future will need to be discussed if you are together again anytime because if you both have different views you may find your better off apart.
maybe she changed her mind because she thought that she cant love you anymore and she don't care for you. maybe she is upset from you. Or she found another boy friend.