Sometimes dogs do this when they are nauseous.
Even though you looked in her mouth she might have something stuck between her teeth, wrapped around her tongue or an infected tooth or even something stuck in her hard or soft palate would cause this. If she will let you try running your finger around her teeth at the gum level all the way back and top and bottom. Feel her tongue and under her tongue. A pin or needle or thread or a splinter from a stick etc can all be a problem that will make a dog drool. Even a rubber band stuck somewhere could do it.
If any of the gum looks red and sore that would do it too.
You can read more on drooling issues here
http://www.petplace.com/article.aspx?id=13
If your dog is still drooling in the morning I suggest seeing your vet about it as you may need x-rays or a full exam to figure out what is wrong.
Hope this helps you!
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