This is a very common problem in older female dogs. It has come to the attention of many veterinarians that spaying a female dog too soon, or while spaying while in heat, can increase urinary incontinence in the dog's geriatric years.
This is also a sign of a UTI; however cancer (in an intact female), stroke, or other ailment can be to blame.
Any time someone has an issue as small as a UTI with an older dog, regardless of health, the dog should be taken to the veterinarian for a check-up.
Bottom Line: Take the dog to the vet, with the above reasons in mind.