Sometimes they don't know or understand what they are feeling. It very probably IS about her gran, but she can't analyse her feelings and identify it. She might be worried abut death - about other people dying - about you dying?? She may feel so overwhelmed by emotions and uncertainties that she has never experienced before that she is having, well, almost a short-circuit, iyswim.
I would suggest that you stop being in her face about it with all the questions and the trying to change stuff (like what you watch on tv at 6pm!!). Instead, say to her that you are always there if she wants you, for a talk, to listen or for a cuddle. And tell her loads that you love her. but accept that she might not be able to give you an answer right now to what is up with her. Maybe stop trying so hard to do something, and just do your normal family stuff.
Part of me is tempted to say that since she takes off to her bedroom and does it there, just turn up the tv and let her get on with it.
oh, just thought - you could draw with her, get her to draw pics of how she feels etc. Sometimes kids can get stuff out in drawings that they can't explain verbally.
If it is that she is overtired, try to put her to bed earlier, like others have said. Have a nice relaxing bedtime routine, bath, supper, cuddle, story, bed. for example.