While I am a cat person, I can tell you of several things she may have. The dog is elderly so she may simply be incontinent. It may also be a bladder infection and the worst possibility is a bladder tumor, which at her age is very possible.

I've been doing a lot of reading on the elderly pet and they can also get dementia. That would also account for her problem.

She needs to see the vet. I wish I could help out more. Good luck