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Assuming the urine is normal and there have been no increase in drinking or urination, you may find that your dog simply cannot hold her bladder. This can be only while sleeping or all day long- the difference being the amount of urine that accumulates while she sleeps. Typically, incontinent dogs "leak" throughout the day- some, only when their bladder is full. As pets age or after they are altered, hormone levels can fall which is why incontinence is found more frequently in older dogs. The hormones help the urinary sphincter constrict- preventing leakage. As the levels fall, they relax- which is predominantly worse when the pet is sleeping and muscles naturally relax some. In many cases, there are medications that can help control incontinence.
I would also mention to the vet that she drinks a fair amount of water. An increase in drinking can indicate a medical condition. Sometimes just an infection can cause an increase while other times it can be an adrenal gland, kidney, or liver issue. Ask the vet what the concentration is. If it is low, Ask the vet to follow up on that. Some of these conditions can go on for years without being detected due to the discreet symptoms.
Here are some links on incontinence you may find useful:
Hope this helps. Please let me know if you need additional information.