in (one's) head

1. Not substantial or real; having been imagined or invented. I don't know why you think I don't trust you. I can assure you that it's all in your head. This fear is just in your head—you're going to be fine. No, I really don't hear any strange noises. I think it's all in your head.

2. Within one's brain, mind, or imagination. What possessed you to let our indoor cat go outside? I swear, I don't know what goes on in your head. After nearly four hours trying to come up with a solution, I finally had a light bulb go off in my head about what we needed to do. Ugh, this song has been stuck in my head for days—I'm so sick of it!