My DS (6) almost always has bruises on his lower back. They're fairly small. Grey, never purple. Sometimes I mistake them for a shadow because he's quite slim and bony. He usually has a few. At the moment, he has one right in the middle, on his spine, and one towards his hip. I never notice if they go away and come back, but every time I seem to look, they're there. Do you think this is something to worry about?