Urinary incontinence is a rare problem associated with spayed female dogs. By this time I would suspect she does have a low grade urinary tract infection due to the constant moisture around the vulva. This will need to be treated with an appropriate antibiotic.
The incontinence can be treated in a couple of ways. We generally try the drug phenylpropanolamine first. This is an antihistamine that will act to tighten the urethra and prevent leakage. If this is not successful we use a hormone treatment of diethylstilbestrol. You do not have to live with this problem! See your veterinarian for treatment.