Etiology of non-traumatic bruising?

My husband has complained of pain just above, and behind the knee.  He also has a substantial dark purple bruise, slightly bigger than a softball.  He has played softball for years and exercises regularly.  Recently he has been painting the exterior of the house and has been on the ladder occasionally.

It is quite painful and he says, "I feel like I am seventy years old!"  He also complains that ambulation is painful.

There has been no trauma to the area.  Can you tell me some possible causes.  He will see his family physician but we wanted to get some kind of Idea as to the cause of this.

[star queen]

Since your husband has been on a ladder painting, I will "guess" that he stretches over the top of the ladder.  This action is balanced be leaning into the ladder.  If he has been painting on the ladder backwards, he would be balancing his weight against the back of the knee.  Another position that would cause this bruising would be if he were placing one leg over the ladder straddling the ladder again balancing with his leg.  It would not have caused him problems for awhile and by then he would have forgotten what he had been doing.  Use ice to decreasing any swelling. Seeing the MD is a good idea since the bruise is a large one.  If the brusing continues or pops up in different places, have the MD run a CBC and Chem Panel blood tests just to sure it isn't something else.  good luck