I Think It's Inside Me

It started with a knock. No one there. No explanation. Just a cold rush of air and a feeling she can't shake-like something slipped inside her when the door opened.
Now, things are changing. Thoughts that aren't hers. Movements she doesn't remember making. A presence that watches from behind her own eyes. She tries to ignore it. Tries to convince herself it's all in her head. But deep down... she knows the truth.
If it's in her mind Why does it feel so real?