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What makes you concerned? What symptoms do you have? Do you suffer from any medical condition or take any daily medications?
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This looks more like a symptom of a bleeding disorder. To diagnose the cause your doctor will take a detailed history asking you about similar episodes in the past and also will do certain tests with your blood. Also imaging studies of your arm may be done.
If the bruising is rapidly growing with referred pain then please do not delay in consulting your doctor. In fact these symptoms do warrant a visit to an ER or an urgent care center.