t been already - as we all know from inadvertently getting water up your nose or inhaling something, that the sinuses do drain down our throat
, so if you have a sinus infection or abscess, it could be draining into your throat
causing the horrid taste. It's possible that if this is the case, you notice it when you yawn because just the act of yawning stretches the mouth and can put pressure on the sinus area, therein possibly causing it to drain.
Are you running any fever? any chills?