What Causes Bruising At The Back Of Knee?

I believe I have pulled my hamstring. I was playing basketball and felt a pop. I was able to walk but I would feel discomfort and tightness while trying to touch my toes. I rested for a couple of weeks and seem to be fine. I went to play basketball again. After playing a few games I went up for a rebound and turned to make a pass. I felt a slight tightness in my hamstring and stopped playing. As the days passed I noticed bruising just above the back of my knee. Is this more serious than just a pulled hamstring?