If what you have is a bruise it might not be anything to worry about ... also if it appeared suddenly. You didn't mention if the area was sensitive or tender like a bruised area usually is to the touch. Just to be safe I would probably have it checked out by your Primary Care Physician .... one can't really speculate over the internet. Regards .....
Was there any recent trauma to the area that could describe this (getting hit, car accident, etc...?) Otherwise, I would call your doctor for an exam.