A pain while stretching

When i was stretching today at the gym i was in a full split (the kind where one leg is in front of the other, stretching one hamstring and the other hipflexor) and i leaned forward and turned my torso so i leaned my shoulder down into my shin. I've done this plenty times before, i stretch every day, weight train every day. Suddenly a small spot on the back of my left calf, the calf i was leaning in to got these sharp burning feeling on the skin. The area was about the size of a quarter, i stopped stretching and checked the area i was stretching on, there was nothing i could be cutting myself on, i did the stretch again and there was the same pain. I worked a couple other stretches then went back to this one and there was no pain. Anyone know what that was?

the stretch i was doing looked like the top picture on this page ( the black and white pic on the left) except i was standing up when i did it
 
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