It wouldn't hurt to take the little one to the vet to have her checked out just to be sure it's not a bladder infection. At her very advanced age it's entirely possible that her immune system is weak enough that she picked one up.
I am thinking more along the lines, though, of it could just be her age and senility catching up wtih her. Dogs are just like people. As they age, they develop little quirks and habits that they never would have dreamed of doing as a youngster. It could be that she just isn't able to hold it like she used to, and the unsettled thing could be that she KNOWS that she should do it outside, she just can't tell you quickly enough for you to get her there.
As long as the vet gives her a clean bill of health with regard to her bladder and kidneys, have patience with her. I know it's a pain in the neck to constantly have to clean up after her, but she has been a loving, loyal girl to you for all these 18 years and she's worth it. You have been fortunate to have her as long as you have, and you won't regret cutting her a wide berth when it comes to being patient with her.
Please come back and let us know what the vet says about her.
Ghilly