Pain in hamstring

Tonight I was running 9 60sec intervals. On the 8th interval about 45sec I got a sharp pain in my left hamstring. I didn't have to stop runnin but did have to slow up a bit. I completed the jog and ran my last interval with noticeable pain.

Any idea of what happened? I don't think I pulled it, but haven't ever had a pulled muscle. I would think it would have hurt worse if I had pulled it.