Went to bed last night and was fine.  When I got out of bed this morning my right knee was in serious pain. Very stiff and hard to straighten out and just a little puffy.  feels like a strained ligament but could just be inflamation.  So I'm straight legging it around today. Feels like a 3 or 4 day recovery (at least)  injury.  
Now I've popped my knees many many times over the years playing sports but this is unusual.  I can't remember anything I did during the day yesterday that would account for it.  I can't believe I did this while sleeping!  
 
Of course this means I can't do much today either. Even going down the cellar stairs to play in my woodworking shop seems unlikely right now.  Bummer...