I have had many dental X-rays of teeth 2,3,4, and 5 from my dentist, and a endodontist. I really do not think it is a abscess unless it is draining before the x-ray is taken. Or I had a abcess that has moved into other areas of the face. I initially thought it was a dental abscess, but I am not so sure anymore. No fever, or chills. I have had some sinus issues for a while now burning eyes, sensitivity to light, one sided sore throat, and phantom tooth pain. All of theses symptoms seem to come, and go on a weekly basis. I have also spoken to a ENT about these problems. The throat he diagnosed as GERD, but the prilosec hasn't seemed to help. Sore throat comes, and goes. If it is a dental abscess I am pretty confident someone would have found it by now. I have talked with multiple doctors, and dentists about the problems I am having including 2nd opinions from some. If I wasn't so normal I would actually be starting to believe I am crazy.
It sounds like an abscess to me. When the dentist checked your teeth, did they take xrays or did he just look in your mouth? If he didn't do xrays, I would return to him and request that xrays be done. I had an abscessed tooth one time that, when the dentist looked at it, he only said it had a bad cavity, but yet when they took xrays, it quite clearly showed a large abscess.
Also have your ears and sinuses checked if they haven'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?