Looks hot! Will try it.
By the way... The menstruation comment makes no sense. Menstruation is the process of flushing out all eggs because they've gotten too old (hence it's the only truly safe time, since there're no eggs to fertilize). If she only menstruates once a year, it means she is fertile most of the time (not the other way around). You need a better excuse, haha...
Maybe you could say she can only get pregnant under special conditions? Like, only on nights with a full moon? Or something else magical? haha...