What Causes Leg Pain Along With Purple Colored Bruise?

I have had pain at the back of my leg yesterday and today while showering a dark purple bruise appeared right at the back of my knee. I didn't bang or hit myself, and the pain is still there in the muscle at the back of my leg, not where the bruise is exactly but feels like deep within the muscle. Worried about heart issues...
any ideas? should I be worried?
I am 34, 5'6'', 80klb, smoker