Sudden bruising on arm. red, hurts when pressed on it. appeared suddenly. no injury or trauma. help?

Get a blood test: Sudden bruising without injury should be investigated with a complete blood cell count. This will include a platelet count as well as red blood cells and white blood cells. Start with your primary doctor, for the blood test. He/she may send you to a hematologist if the test indicates and abnormality. Or maybe you just are an aggressive sleeper. Don't ignore it, simple blood test