From the Pasuk that Coffee Addict mentioned it seemed to me that obviously Leah didn’t know. There are Midrashim about Leah answering Yaakov Avinu in the morning that she learned from him how to trick people, but such Midrashim might be conveying an implied conversation rather than a real conversation. The Medrash says that he had in mind to divorce her but when she had children he kept her. In the end, he agreed to it as it says ?????? ????? ?? ??? ????.
The reason Rachel went along with it was not to embarrass Leah. According to this understanding, that Leah wasn’t aware of this being a trick, the embarrassment would be tenfold.
With this we come to understand another topic. Chazal tell us that when Rachel gave away a night to Leah in return for a plant, she lost her opportunity to be buried with Yaakov. “She made light of being with the Tzaddik so she wasn’t buried with him.” The question is, why indeed did she give up being with him for a flower?
However, now we can understand. Leah told her, not only you took my husband you also want to take what my son brought me? This can only be said by someone who really thought she was cheated of what was rightfully hers. Rachel went through her life acting as if it was she who cheated her sister. Now, faced with an accusation of attempting to cheat her again she had to think to herself how someone who is actually guilty of that would respond. Therefore, she answered, tonight can be your turn.