What Does Black Bruise At The Back Of Knee Indicate?

Hi, I exercise about 3-4 times a week but about three week ago my knee started hurting after I did some running and changed weights. I decided to stopped going to the gym until the pain subsided and started going 2 days after but the pain continued coming back. After two weeks of not doing any running or spinning I decided to try it again, since the pain was gone. During the spining class I felt a bit of discomfort and then I noticed the back of my knee was bruised, purple almost black bruise. What can it be?