Who's Sarah?

I think Avery has an imaginary friend. That or an identity crisis. She has this headband we put on her at mealtimes to keep her hair out of face and tonight, after dinner, I tried to take it off and she got really mad at me. She said "NO!! Then I'm not Sarah!!" I asked her to repeat herself, and she said the same thing. I put it back on her and she said "Good, now I'm Sarah again. What's your name??" I said 'Mommy' and she said no, you're Mommy Sarah. Whatever that means. She has mentioned 'Sarah' for the last couple days here and there, but this is the first time I've really paid that much attention to it. She just sat down on the couch next to me and yelled "Sarah!" and then jumped off the couch headed to her room. I asked where Sarah was and getting the 'you're and idiot' look, she pointed to herself and walked off saying it was her turn at the doctor's office. She just got Benjamin to call her Sarah as well. She has always had a great imagination, I guess this was coming. Weird thing is, we don't know any Sarah's that she sees regularly. Elijah has a cousin named Sara that stayed with us back in the early summer, so that may be it, but otherwise I have no idea!!