Strange girl in my classes talks to herself?

Basically, there's this girl who I sit next to in most subjects (Because our surnames both begin with s, it's alphabetical) and she basically just...mutters to herself all the time, and I don't know when she's trying to talk to me or when she's talking to herself so I tend to ignore her, but I feel bad, but she's so weird, like she'll narrate everything she's doing! Like lets just say that my names Amy. The other day, I had my feet on a chair opposite me in the library (reading a book), and she came over, and just stared for a while(I didn't notice her) and then she started talking to herself! saying "I was going to sit down on that chair. But I can't sit down on that chair because Amy's feet are on that chair. I am going to go find another chair." It was only me and her there so she couldn't of been talking to anyone else! She's so strange and she always follows me around, what do I do?!