What makes you think Lea was aware of the Nemfection in SK?
I was talking about Maeve.
We do not know when Lea became aware of it but it could have been from the very beginning but not before she got it because she called it a treacherous gift.
However after she became aware of it she tried to fight it alone. I think she infected Maeve before she became aware of it or at least when nemesis was in control so pretty soon after she was infected herself. Lea did not want to run to Mab until the shame became too much because the athame represented a lot of power.