What Causes Bruising At The Back Of Knee?

I have brise that I notice developed saturday morning while shaving my legs behind right knee. It looks older in coloring (brown/slight yellow, but more faded brown like it's older) and I don't remember hitting the back of my knee with in the last week. Its sore to touch and is tender when I straighten my leg. I want to a pain relief for it but am not sure if it's something more.