Why is part of my thigh muscle numb?

On the top part of my left thigh, in a spot about as big as a hand, my skin and muscle is numb and I can barely feel it. I don't know what happened. I'm not one for strenuous exercises, although I do practice yoga. I'm also on my period. Could I have something to do with that? I don't know what's wrong and it worries me. Please help!