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Your bruise could be due to a musculoskeletal injury, which you may not have remembered.
Musculoskeletal inflammatory pain can be diagnosed if the pain increases on specific movements or on direct pressure on the affected area.
Such cases of mild pain, subside on its own with rest. If needed anti inflammatory medications like aspirin or Ibuprofen 400 mg can be taken upto three times a day, to relieve the pain.
In case, the pain and the bruises recur again, you might consult with your doctor for a examination and proper advice.