Could be anything. Could be a muscle knot, could be a cyst, could be a benign growth of tissue, could be worse. Could be something else. Get to a doctor. You need someone with experience to examine it.
It could be anything but most commonly it is a lymph node. This would be right under where your jaw hinges. It may (or may not) feel hard, warm to the touch or painful. If it lasts for more than two...
I knew an old lady(94 yrs old) who has now passed away of old age, that had that exactly but it caused her pain, was always there since a fall that caused an injury that caused calcium to grow called...
They can be mineral deposits from your body (mine were caused by caffine) or something more serious; See you doctor.
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They could be swollen lymph nodes. Lymph nodes are grouped...