Someone's story appeared in My Stuff that I didn't write or take part in in any way even though it says I wrote it.
Have you shared your password with anyone?
But it doesn't say who the original author is then.
Is their a checkmark in that storygame's row?
If there is, someone coauthored with you. Click on the storygame title to bring you the storygame page. You can find the original author there.
Have you used public wifi recently?