Dog shadow boxes

Just recently within the past month, my poor dog has been aggressive towards her shadow?! She was always like that with light reflections on the walls or ceiling. But now, mainly at night, she has about three spots in the house where she barks, scratches, pounces and even licks the floor where her shadow is?! I feel bad for her because this could go on all night. I try to keep her occupied, but i cant play with her all night everynight. Also its currently freezing outside here in NY so leaving her out is not an option. Any feedback as to why she does this and/or how i can treat it would be greatly appreciated.