Has anything happened recently that could have made her afrid , any new happenings ? Could she have seen something scary on TV or have any people in her life been playing a loud, noisy game with her, it doesnt take a lot to make chilren around this age afraid of what we think of as nothing...Dont over react when she freaks out ,make no issue of it as sometimes it can feed off itself ...You say you try to reason with her, so try not making any biggy about it ...it will pass..