The Vilna Gaon points out that “ma’amar” (“instruction”) is a word connoting a gentle form of speaking. Esther, being queen, still followed Mordechai’s instructions like a daughter following the gentle reproach of a parent even after leaving his authority. Rashi adds that Mordechai sat at the king’s gate (see next verse), gently reminding Esther that she was Jewish and thus had responsibilities to H-Shem.
The Talmud (Megillah 13b) says that Esther showed her niddah questions to Mordechai, as it may be necessary at times to seek rabbinic advice in this area according to the halachah (Shulchan Aruch, Yoreh Deah 193:1). In fact, the Yerushalmi states that Esther asked Mordechai other questions in regard to rabbinic law. In other words, she was asking specifically issues of rabbinic law.
Rabbi Dovid Feinstein says that Esther was following Mordechai’s advice because she knew she was a part of H-Shem’s plan, but did not know how, and did not want to get in the way of this Divine plan.
Rabbi Tzvi HaCohen Kaplan writes that Esther’s prophecy mentioned earlier (mentioned previously) was a direct result of her fidelity to Mordechai, trusting the Torah of the Rabbis in these difficult times.
Another opinion in the Talmud there is that Esther had relations with Achashverosh, went to the mikvah, and then had relations with Mordechai. Although she was taken by force, and so allowed to Mordechai, Esther seemingly should have waited the requisite three months (Yevamos 35a) before having relations with her husband, unless it was actually a sheid (as mentioned in previous posts) having relations with the king. Perhaps going to the mikvah was Esther’s way to feel more pure, even if she wasn’t so in actual fact.