I've noticed this same bizarre thing happening to my Ranger over the last few days too. Not just this, but I will find myself suddently APPEARING in Stealthed Mode (Invisible) even whem I'm NOT Stealthed (I know I'm not because I'm still running at full speed and attracting monsters).
This weird backache thing is the worst though because it makes it difficult to run around until it clears up 
Great picture by the way - Definitely made me laugh :smileyvery-happy: