I've had this happen only a few times over the years. Not bad enough to see a Dr. but I did try to find info on it. The one I remember was that it could be due to excess iron in your system. At the time I had been living in an apartment that had well water and it seemed to happen after eating vegetables (the green iron rich kind) so I though there might be something to it.
I can suggest looking this up online for your own comfort and self education. It could be as simple as mouth hygiene needs or sinus problems even. Even medications can have side effects. IF it continues see a dentist or doctor. Try maybe rinsing your mouth with some kind of non irritating mouthwash and see if that and an extra brushing might help. I am not sure, but for now maybe it might at least give you some relief. I hope you get the help you deserve.