We had a cat who lived in the cellar until I moved in.  I trained her
to be an "upstairs" cat.  She never really socialized (big surprise),
but then she got to be so affectionate.

Anyway, when she was still confined to the downstairs, she'd go out in
the yard, sit in the sun, lay on the deck, etc.  Sometimes in the
summmer and even the winter she'd disappear for days on end and come
back no worse for wear.

Now, she won't even go near the door.  When I open it, she runs away.
She's spending her whole life in a dark closet sleeping, and she wakes
up at night only to eat, be petted, then go back to sleep again.

Now and again she'll zoom at 3 or 4 AM, pouncing on invisible things.
But that's only once in a great while.

I am SO sad.  I created a monster!  This formerly happy, if chilly and
underappreciated little cat, has gone to being a fat, lazy, weirdo.
It's so sad.  I know 13+ isn't young, but it's not exactly one foot in
the cat grave either.

Is it possible this is a behavioral thing and she's not sick?  Could
it be that she got spoiled being an upstairs regular housecat and
avoids the places and behaviors she used to have?