Thanks for all the replies, she had a checkup about a month ago and the vet said she was (despite her age) in good shape. On the chance it could be aches and pains, since she's a pretty stoic dog, I gave her a tiny aspirin before bed and she's slept quietly two nights in a row. This might be the solution, but time will tell.
I still believe she's got her wits about her, she usually acts confused only after it rains and her "map" of the property is erased. A day later, she's back to it.
I'll keep you informed.