Put yourself in her shoes. This is like the good film, the good day out, the good book, the good gadget, etc. How many times in your life have you say, read a good book and then followed up either by seeking out other books by the same author, or on similar books by other authors? Sometimes people can amass small libraries of such material then maybe after a short time lose interest in it - other "shinies" come along to preoccupy the mind... and that's it! Maybe after a year or so you might pick up one of those books and the cycle goes through once again, maybe weaker, maybe stronger.
My guess is that, yes, you were special, you made an impression on her and she genuinely wanted all your babies at the time you met with her. The following day she met someone else who had a similar effect on her - don't forget she is in her twenties, emotionally she is still relatively malleable. I dare say that, yes you will hear from her again, when she perceives maybe that she has come to the end of a particular road and wishes to explore another avenue. Probably this will be as a result of a bust-up with another man/punter who has succeeded in getting a bit further than you.
So what will you do if/when you see her number coming up on your phone? Has she changed her mind? Is this "and they all lived happily ever after"? Not so much a change of mind, her mind flips constantly from potential commitment to potential commitment, yes she's after a fairy-tale ending, but she can't grasp with who, and she probably never will, unless she meets someone who is controlling and assertive - which will probably signal "game over" as far as her "fairy-tale" dreams are concerned. If you meet her again, and I reckon you would say yes given the opportunity, it is going to be a roller-coaster ride, she could very easily destroy your marriage, and then disappear again for another long period. Unless you are extremely self-disciplined, have the skin of a rhino and thoroughly understand the dynamics between you, her and the multitude of other lovers that come and go in her life, that you will never ever pin down this woman, you would be advised to steer well clear.