you can be more sensitive to everything during your period including pain as well as emotions, dieuretics taken regularly 2 weeks before the start of your period makes a big difference as it helps reduce the water sustained during that time which causes the,sluggishness and bloated feelings, tissue salts are also supposed to be very good at relieving period problems as they restore the bodys natural balance of hormones mnerals etc, they're totally natural are safe and have no side effects, ive had them recommended to me by a few diferent women including my gp though ive not yet got round to trying them lol.
if you're not comfortable with sex during that time greendragonfly then to keep the D/s side going why not come to some compromise such as perform oral,be restrained, adopt certain positions, obey orders, dress a certain way etc while he or you masturbates him,
perhaps you could both try to experiment with various types of play maybe a role play type scene...a serving wench fetching his drinks..school girl doing homework..or whatever lol (you might be surprised and find there's some things you're happy with) that doesnt involve full on sexual activity or expose your body but still pleases him.
that way he's still 'controlling' the situation (to a degree) and you're not uncomfortable with having to have sex.
and if nothing works then i guess you both have to accept that and put it on hold for those particular days, it would be pointless being tense and uncomfortable as that will just build into frustration and resentment on both parts if its not handled carefully.