its because when you have a period the amount of serotonin in your brain produced decreases and chocolate contains natural serotonin. Serotonin is what makes up feel happy, a lack of it causes us to feel weepy when we are on, its what leads to dpression.
i'm on at the mo and have been fighting the urge to eat chocolate but all this talk is making me crave it all the more!! now wheres that left over easter egg...
stylinsam that happens to me also. and the funny thing is i would have all the symptoms for my period but it wouldn't come unless i satisfy that craving.