Calf tightness

Hey everyone, 

I’ve been running for some time now never had this happen before. My left calf has been really tight couldn’t even get my girl to massage my leg that’s how tight it was. Now it’s better but whenever I run my left calf still feels tight, and in front of my left leg on the left side it goes weak. I can stop walk a bit to stretch it out and run but then again it goes weak. My calf then started to hurt when I was running downhill and behind the knee. That’s when I decided to stop running. No clue what it is, any ideas? Thank you