You'll need to take your dog along with a fresh urine sample to your vet. If the urine will be more than an hour old when you arrive, keep it in the fridge until you depart.
Our dog did the same thing out of nowhere and turned out to be kidney failure. I'm not saying that's what your dog has, but it needs to be investigated. Another possibility is a simple urinary tract infection that needs a round of antibiotics to cure.
As Chirley mentioned, it could be simple age-related incontinence that can be helped with medication, withholding water for 2 hours before bedtime, and making absolutely certain she urinates before bed.
Your vet will definitely need a urine sample, and should also do some blood work in the lab to determine exactly what is going on. Please don't put it off and let what may be a simple, inexpensive treatment become an expensive, life-threatening illness.
I have had 2 female dogs that have become incontinent of urine as they aged. The treatment was easy, effective and cheap. The vet started them on a course of stilboestrol (sp). If memory serves me correctly they started off with a tablet every day for a week and then reduced to a tablet every 5-7 days. It worked very well.
Hope this helps.
Chirley