Thank you very much for the useful information Richard.
Most likely this is a result of trivial trauma to the temporomandibular (jaw) joint, or its associated structures (like the muscles which move this joint).
Yes rest is fine, but along with that you need anti-inflammatory drugs.
Take Motrin or Aleve which will not reduce the ongoing inflammation, but will also help relieve the pain to a great extent.
Apply heating pads to the affected area (heat itself is one of the best muscle relaxants and gives a soothing effect).
Take soft diet (so that you dont have to chew the food forcibly).
See how you respond to these measures in next 48 hours.
If the problem persist, does not improve or gets worse, you need to see your doctor for a clinical examination and X-rays of the joint to make sure there is nothing wrong like a fracture (which can be minute/hairline) and can continue to give you pain due to impaired healing unless treated otherwise.
I would have been worried too if I were at your place, but let me reassure you that this is a treatable condition and carries a good prognosis with appropriate management.
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