Why is my niece so scared?

My niece is 19 months and recently started this thing where she gets frightened by something completely randomly. It started with looking down a hall that wasn't dark and then clinging to me for dear life and crying. Then she wouldn't look out the window at her house which she typically loves to do. And now my sister said she crys and clings on for the wierdest things. She said "the phone is ringing who is it" to my niece and she freaked out and started crying hysterically and holding on to her as tight as she could. It's so wierd and random. What could this be?