Yesterday worked arms, started with heavy barbell curls, then did heavy alternating dbell curls and then seated hammer curls. Moved to tri's started with v-bar pressdowns, first 2 sets were light then started bumping up the weight. I was really feeling the movement, which is unusual and right at the end of my last set I pressed to extension and WHAM, I had some sort of crazy contraction, one I've never felt before. When I walked away from the machine, my left tri was aching really really bad, and within 3 minutes had blown up like crazy. It was probably twice as pumped as I've ever felt, hurt like hell and I couldn't bend my arm. Went to do some dbell behind the head extensions and couldn't, the pain was unbearable. My left tri was just insanely blown up and it really really hurt to move my arm. I decided to play it safe and quit, today my arms are fine no bruising my left tri is really really sore what the hell happened?