behaviour change from happy to scared???

My little much loved jack has gone from being a happy go lucky girl to hiding behind the sofa and being very quiet and clingy to me.
Ive had her from a pup she is nearly three, nothing has changed in her life my daughter came home from uni but she knows her so i dont think this is the problem. her food has not changed she is not sick and she get a walk everyday in the park which she loves! anyone knows what has happened please let me know Id be very grateful!