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Hello. Thanks for the question.
The main worry I would have here is a condition called MMM (masticatory muscle myositis). This is an auto-immune condition where the muscles tense up and eventually fibrosis sets in. The initial signs we see are pain when opening the mouth.
The diagnosis is typically made during examination. A muscle biopsy can be done to confirm the diagnosis. Treatment is then with immuno-suppressant steroids (Prednisone) +/- other immuno-suppressant meds like Imuran (Azathioprine). A full recovery can be made, but it does take some time to get a response.
Here's a nice link on the condition:
I hope this helps, let me know if you have any other questions.