Pressure differential in your maxillary sinus on one side. See your dentist or physician for relief. Yes, very painful, indeed!

Can try vasoconstricting nose spray, e.g. Afrin or Neosynephrin. May work faster than finding a dentist or physician on a weekend!

My sincerest thanks for this advice, as well as to others who suggested it might be a sinus problem. The pain had gotten really intense, and I remembered that I did have some nose spray upstairs. Two squirts up the side with the pain, and the piercing pain went away almost instantly. It is still a little sore, but before the nose spray it felt like I was undergoing a root canal with no anesthetic.

You're more than welcome. Glad you tried it. Often the simplest solution is the best.