Multiple Personalities Support Group
Dissociative identity disorder is a diagnosis described as the existence in an individual of two or more distinct identities or personalities, each with its own pattern of perceiving and interacting with the environment. At least two of these personalities are considered to routinely take control of the individual's behavior, and there is also some associated memory loss,...
I have heard of this kind of thing, from at least two therapists...it's called an "abbreaction"...a flashback manifested physically. I have had a severely itchy rash on my forearms suddenly appear and then a few seconds later, disappear again....(I happened to be reading a book that mentioned this phenomenon a short while later in the chapter)
I've also heard from my nursing professor, (who I respected and admired...she is an awesome person) when I asked her a very similar question to yours, that it is an alter who switches and does the injury, and that's why it seems spontaneous. However, that does not address the sudden disappearance again.