At this tender age, I personally would not make her suddenly sleep alone
.
She is likely experienceing some mental changes / dementia. You should really explore giving Anipryl for this - it can make HUGE changes in a dog's quality of life!
As for the excessive driniking -- trust me -- there is *always* a medical reason for this and if your vet can't find it - I'd look to another vet. Excessive drinking/peeing should always cause a vet to rule out diabetes AND Cushings.