Purim

Megilla 15b-Rav Nechemia- The reasons that Esther invited Haman to the party was so tyhe Jews don't think that since we have a Jewish queen we don't need to daven. In Esther 4,16 Esther tells Mordechai to tell all the Jews to fast for her sake. Clearly all the Jews knew she was Jewish. If they all knew she was Jewish, how come Achashverosh didn't find out she was Jewish?