A gal can definitely change her mind. However, in ur case, u
said she's a shy and friendly gal. That's the problem. A friendly
gal will definitely be very nice to everyone ard her, sms/msn or
meeting frequently don't really mean much. Furthermore, she has
rejected u once. There may be a possibility that she thinks u have
already taken the hint and move on, and hence lower her guard
against u, not knowing u still like her.
But of course, I am not her so i don't know. All I can say is take
things one at a time, try 2 find out from normal conversations if
she's seeing anyone now, or if u are slowly becoming an impt part
of her life.