There is a fascinating Daat Zekenim which explains the answer to your question. He writes that the Simanim -"signs"- that Yaakov and Rachel made to be able to confirm that Lavan would not pull a "switcharoo" on them and substitute Leah for Rachel on the wedding night (Megillah 13b) where actually the laws of Niddah (women's menstruation cycle), Challah (separating bread), and Hadlakat Haner (Lighting the Shabbat candles). Thus, when Rachel feared that, should Lavan actually substitute Leah for her, Leah would become embarrassed if she didn't know the Simanim (see ibid.), she gave over the Simanim to Leah. However, says the Daat Zekenim, Leah did not know these were Simanim; she thought Rachel simply wanted to teach her some laws! And when she "passed the test" of knowing the signs when Yaakov tested her, she assumed it was a mere coincidence that she had learned these laws previously with Rachel. Thus, it turns out that Leah actually never knew of this whole scheme, and of Rachel's sacrifice for her; she honestly believed that Yaakov was originally her husband! It is this that led Leah to exclaim "it's not enough that you took my husband, etc."
(Rachel, however, did not answer back and tell Leah the whole sacrifice she actually made for her. This was the Tzeniut -modesty- of Rachel mentioned in Masechet Megillah 13b.)