Renkma, no doubt you've gotten your answer by now, but my dog went through a similar thing this past year. She's always had a touch of kidney problems, but late fall 2010 it was as if she'd become incontinent. She was making it outside most of the time, but was also having major accidents, wetting her bedding, and lastly leaking on her bedding at night while asleep.
At age 14 at the time, I assumed she was just getting too old to hold it, but I did take her to the vet, and much like your dog, her blood work and urinalysis were ok, aside from dilute urine and the ph or creatinine (I can't remember which), were off.
Vet said in all likelihood she was in early stages of age-related renal failure, so onto a lower protein diet she went.
But what made the BIGGEST difference was her being on estrogen supplementation. She was given a booster of pills daily, and now she takes it once a week. This corrects spay-related incontinence and I can tell you it was like night and day with this dog. She has now been on it several months, and while I can't say she's never had an accident since (she's 15.5 now), it made SUCH a huge difference to life around here, and spared my state of mind too because I was constantly washing bedding and cleaning floors.
Someday I hope to be the kind of person my dog thinks I am