I'm sorry Joan.
I have sinus problems, so nose is stuffy most of the time, so mouth hangs open.
..just like that.
Could that be your reason?
I use sinex spray to upen my nose and close my mouth. I have to think about it too...habit.
Dry mouth is a result and that's bad for your teeth.
I hope it's something you can remedy soon and it's nothing more serious..