Brief Answer:
Temporo mandibular joint problem or trigeminal neuralgia

Detailed Answer:
Hello. I have been through your question and understand your concern.

According to the symptoms you are describing, I see two options.

First, temporo mandibular joint inflammation, where the pain should begin while opening the mouth and persist even after it. It is a very annoying condition but totally benign. RX of the region, helps with the diagnoses, while NSAIDs help recovering. It might get chronic, so you need a maxillo facial expert consult.

Secondly, trigeminal neuralgia is a condition where the pain comes spontaneously and is sharp ( if dull, you need a more exhausting neurological monitoring to settle the diagnoses, but it is a benign one, which comes and goes) sometimes called lancinating, which seems like you have been cut with a knife) Some conditions precipitate the pain, like teethbrushing, shaving. In this case a neurological consult is recommended who will decide if Cerebral MRI is needed. Surprisingly antiepileptics work very good in this case ( carbamazepime especially)

Hope this helps.

Greetings

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Brief Answer:
Middle ear involvement is possible

Detailed Answer:
Hello again.

Facial pain can also come from ear structures involvements, like inflammation or infection.
It definitely does not seem like a sinusitis, because it has not the characteristic features.

If middle ear gets inflammed it pushes the inner structures and causing irradiating pain to the lateral side of the face.

The diagnoses is simple because with an othoscope you can see the bulging of the ear drum, suggesting middle ear involvement.

It is not dehydration as well.

You need a physical exam of the area to rule out these conditions.

Wish you the best health

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