Are you eating enough? I've found that when I don't eat, or not enough, I get bruised when I sleep, on my hips and the inside of my knees where they press together, as well as my lower legs in general.

If you are eating enough... maybe you should try eating foods rich in iron? I know that anaemia causes easy bruising, so perhaps you are slightly anaemic? i don't know really, just suggesting. You could try taking multi-vitamins too and see if they help any.