You told this story with great detail; one could easily imagine the events. You also added sensual perception what made things even more graphic. The story started rather normal. I liked that you used the conversation with her daughter before Sheryl tells about the actual event. It's like she tells her personally and so the reader feels.

I enjoyed her faked fear and teasing with her sister. Nothing hinted at her encounter later what made this last scene even more creepy. I'm not sure of what to think about the woman in the bathroom who went at Sheryl.

Maybe she was really blind and dressed like this purposely (you know, like Star Trek fans at Trekkie conventions) but Sheryl confused by her appearance considered her a real monster. Maybe she even was an acquaintance - why else should she know Sheryl's name? A little child can't yet really distinguish between imagination and reality and they have a really lively imagination. Maybe that played a trick on her. She's totally shook up and definitely feared for her life and that was what gives one the chills.