I'm a ninja ;)
...Or am I? Maybe you made a clone of yourself and I used my Thoughts-B-Gone to erase your memories, and then I reconfigured the machine to implant false memories and made you think you lived in [insert hometown here] and your name was [insert name here] and you had a lifelong dream of starting a peanut butter and jelly sandwich-making factory! In the meantime, I posed as you and took photos and wrote details plans about your house, your life, everything about you to one day get rid of you and stand in your place! But when your family was getting suspicious, I swapped our memories (only the safe, personal ones of course--none of my plans) and returned to my home planet to plot my revenge!!!
*cue epic evil laughter*