Back Scratch Fever

Some of you will remember this post, from last March, in which I showed a photograph of mysterious scratches that appeared on my left shoulder.  I bite my fingernails and they do not protrude above the flesh of my fingers, so I question whether I could have done it to myself.

Well, almost exactly one year later, the scratches have returned, again to my left shoulder:

Any ideas on what this could be?  The logical explanation is that I’m scratching myself in my sleep, even given my stubby nails.  Of course, others will suggest demonic visitation.  Who’s got some real theories?