What Causes Bruise On The Back Of Knee With A White Spot?

I have a large bruise that appeared randomly on the back of my left knee with a white spot in the middle and no recent trauma. The ER doctor had me get an ultrasound to make sure it wasn t from a blood clot but didn t give me any other explanation of what the cause of the bruise could be. It has been 4 days and hasn t gone away. What could be the cause of the bruise?