Probably best to get it checked out.There are loads of reasons for bruising to occur,many of which can be ascertained by simple blood tests eg low platelet counts following viral infections,coagulation problems,liver conditions etc.Most are easily treatable.Often bruising can occur due to bumping your legs when you sleep, etc and more often than not it poses no health problems whatsoever.Some people are just susceptible to bruising.I know I frequently find some spectular bruises and have no recollection of how I got them!