I get that sometimes, i get bruises without knowing how it got there and as i don't remember myself banging into something, and sometimes it doesn't hurt so you don't realise that you have one until you have a bath or something. I had a massive bruise just above my elbow, it was so massive (slightly bigger than an egg) and the funny thing was that i hadn't a clue where it came from.
Not meaning to be rude or personal - but how old are you.
My GP told me bruising easily is something that often happens to women during the menopause.
Along with all the other horrid symptoms!