What Causes A Dark Purple And Green Colored Circular Bruise On Upper Underarm?

I have a large 2 inch circular bruise on my upper underarm. It is dark purple and green throughout but the center is raised, red, and circular about the size of a pencil eraser. It does not hurt and the red raised spot just appeared today. Is this just a bruise or could it be something more? I am a 29 year old male. 6', 250lbs