Bruising would either be caused by typical impact trauma or something else more obscure/recent. Either way, if it is notabld in size or shape, you should get a doctor to check it out. Spinal injuries, or distinct injuries along your spine, are not to be taken lightly. It is especially important to get it checked if you have preexisting spinal abnormalities, no matter how minor they might be.
Is the bruising consistent? Is it painful? Does it cover a large area along your spinal column?