This prompt is a little different. The creative push is from the message: “You aren’t who you think you are.”
Ist person or 3rd, the main character receives this message. How the message is delivered will drive your story: it could come from a relative, a doctor, a lawyer, a cryptic phone call, a voice in the night, a certified letter, a phantasmagoric event or even something supernatural.
Who the character is and who he finds out he/she really is will be your choice.