What Causes Bruise From Knee To Thigh?

I have a large bruise that appeared overnight with no trauma. Its runs vertical from my knee to mid thigh. Its about 1 1/2 inch wide and about a foot long. Its a very black and not terribly painful. One spot where it begins is more sensitive and feels like a small knot. The leg below the bruise is very cold.any ideas on the cause of this would be appreciated