Getting stiff during sex

My boyfriend and I were having sex and everything was going well, and the all of a sudden my body (mostly my hands and forearm) got all stiff and I couldn't control it....

whenever he stopped I couldnt undo my fists, it felt like they were stuck or something and my whole body was tingling.... what just happened?

I didn't drink much water and I only ate one thing that day so I'm thinking maybe I was dehydrated or just hungry? I'm not sure. Answers please??