It contains a small amount of tryptophan, which enables the brain produce serotonin. Serotonin is a chemical that elevates mood. Chocolates also contain high numbers of flavanoids, which protects the heart. To get more effect of chocolate on our bodies, visit http://www.ehow.com/facts_5840422_effect-chocolate-body_.html.
Chocolate has no effect on your period at all. Chocolate has a bit of caffeine in it, but you would have to eat a lot to feel the stimulant effect.
That being said, it has been my experience that there is some kind of special "relationship" between women and chocolate. Women often have a craving for chocolate during their periods that must be satisfied, and that obviously doesn't happen to men. It usually manifests itself when a woman is in her thirties, and I don't think it ever goes away, not that I would want it to.
Personally, I consider chocolate to be its own food group, a necessary part of a happy life. It doesn't take a lot, but it really has to be there when the craving comes. In other words, like many women, I love chocolate in a way that I don't feel at all for any other food, and I don't even consider chocolate to be "candy." I can do without candy forever, but I truly believe that chocolate is necessary for a fulfilled and happy life. :-)