I have no doubt Mab was referring to Lea when she said "she" challenged her authority. But a challenge to Mab's authority does not require open rebellion. Lea attempting to act without Mab's consent is a challenge of her authority. Lea acting in Mab's interest in a way Mab wouldn't approve of is a challenge of her authority.
The text tells us Lea tried to do something against an enemy too powerful for her, she turned herself in to Mab, and Mab helped heal her while reasserting her authority over such things.
I think Griffyn612 has the right idea. What I want to think about further is what Arjan brought up, what did Lea mean by "what stalks us all."
With hindsight I translate "What stalks us all" as Nemesis. Lea tried to overcome it herself but failed. Ashamed she went to her queen.
This reading makes sense to me, and strikes me as the most obvious interpretation. Lea tried to struggle against Nemesis on her own, but was unable to win the fight without help. But the struggle alone doesn't seem like the offense that Mab spoke of in Changes. So what was the action that Nemesis was attempting to cause Lea to perform? A simple, small action of verbal challenge, a simple questioning of Mab's orders would probably be enough to fit the bill, that's most likely.
But what if we think bigger? What if "I thought I could overcome what stalks us all" refers to death? That phrase is certainly a common motif of death. And the goal of vanquishing death is already held by Cowl (according to Kumori anyway), who provided the athame to Lea via Bianca. Lea would be in a position to potentially make some type of move against Mother Winter, who is the closest personification of "death" in the DV so far. And that would certainly qualify as a challenges to Mab's authority, an action against Mab's word and will. If Lea recovered herself and broke down in shame before she actually took an action, then everything fits.