What Causes Pain In Left Knee Followed By Bruising?

I never had any serious injuries before but I am 6"2 and I have been having a really bad pain in my left knee. The last couple of days When I would awake from a short nap even, my knee felt like it was twisting to the left. And it hurt extremely bad, even when I would slightly bend it. But the pain wouldn't go away until I would fully bend my leg in. And afterwards when I touch down on my my knee with my fingers it feels like its bruised. Can you plz let me know if its something serious? Cause it seems like it has been happening a lot lately the last couple of days. Thanks