My friend developed a very large bruise around his elbow a couple days ago. It doesn't hurt at all and he has no recollection of doing something that might've caused it. He was tested for blood clotting problems but they came up negative/normal, and the doctor didn't have any other suggestions but said it will get better.

He feels fine but is obviously concerned about a large, painless bruise on his arm that seems to have appeared spontaneously. I would say it is about 5 inches in diameter and circles the elbow, which does not appear to be bruised itself. He doesn't have any other bruises either and doesn't have any known medical conditions. Anyone experienced or heard of anything like this before?