It is possible that she got cold feet. Of course, she could give you some time frame to keep in mind. Like "oh I got nervous and I need 2 months" or something like that so you know what to expect.
If she seems confused to you as to what she really wants, then couple's counseling/premarital counseling can be quite helpful with that.
Otherwise, you would have to talk to her and she has to be honest with you. People have problems when they are not on the same page emotionally and in their expectations of the relationship. Try to find out where she sees the relationship in 6 months, 1 year and 2+ years.