Thanks for coming.
I am a double board certified US doctor and will help you with this.
It is not necessarily normal.
It can be from gum disease.
It can be from sinus infection.
It can be from acid reflux [this is a common cause and often not diagnosed.]
It can be from a problem with smell as well, but this would not really respond the way you are describing.
I suggest you go over this list with your doctor ASAP.
Not an emergency, but might need treatment.
OK, so you might have more questions or want to give me more information: Please use reply to expert if you have further questions.
Also, please click a positive rating [hopefully excellent (5)—that’s how we are paid, per rating]. If you forgot something, just use reply and come back. I am here http://www.justanswer.com/medical/expert-dr-thomas/