What Do Black And Blue Bruises Behind The Knees Indicate?

The other day I noticed very bad black/blue bruises behind both of my knees for no reason. It's been 3 days now and they still look terrible. The only thing I can think of is that I was kneeling down to fix something that morning. Can you tell me why this would happen and how I make the color fade faster? I am 55, 5'4", 140lbs, arthritis.