Have you ever used it for punishment? If so then that would be the cause right there. Also dogs can be scared of such crazy random things like are the garbage cans right out back where racoons get into them and she can hear? Is there a tree branch that scrapes longside the house?
It is also a natural instincts that if a dog is apart from the pack then they are as good as dead but you say he didnt have a problem before.
Anything different with the changing season's like mice in the air duct?
Has she switched beds?
Taken out toys?
Left the dryer door open?
I know those things sound stupid but something small could be triggering it.