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.
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 .....