There are a couple possibilities in a geriatric kitty like this.
A bladder infection is a common cause of urinary incontinence in older cats. There's no home treatment for something like this, although many websites will profess to have the cure, usually in the form of cranberry juice or tablets. These ‘cures' don't work in animals or in humans. They may contribute to a higher acid level in the urine and more hostile environment for the bad bacteria, which may prevent bladder infections from taking hold, but once the infection is there, it needs a genuine treatment.
This will usually be a course of antibiotics based on the results of a urine culture. It's vital that you persist with the antibiotics for full length of time prescribed and submit to a second urine culture to be sure the infection is cleared. Otherwise you'll not only be facing a relapse of the condition, but it often comes back worse and with a resistance to the antibiotic. A relapse can not only cost you double (or more) in expense, it can be just terrible for your companion to suffer.
Find out more here: http://www.marvistavet.com/html/body_urinary_incontinence.html
In a well taken care of cat like this, at this age, you might also want to consider having a T3/T4 blood chemistry done to test for thyroid disease. It's not unusual and in many instances may be addressed with a diet modification.
This link not only has more info about that, but recipes you can make at home
Diabetes may be another possibility.
All this said, chances are pretty high that it's a bladder infection and it's very important to have her seen tomorrow and get the appropriate meds started right away.
You've done a great job with her - keep it up!