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Have you remembered being bitten by any insect. You could have hurt yourself probably sometime earlier. It is probably collection of blood under the skin due to trauma. Apply cool ice pack of frozen peas with proper insulation to get relief from pain. Consult your Doctor to rule out infection, skin necrosis
, and if haematoma
it will usually get absorbed on it's own if not very swollen. Avoid high carbohydrate
diet and fatty food. Keep your blood sugar in control.