Nimue shrank back a bit when Mordred mentioned her secret. She'd thought she was protecting Mordred at the time, in not telling Merlin. She hadn't wanted to worry Merlin any more than he was. She'd wanted Merlin to think Mordred was a normal boy, not capable of such darkness. But there had been a selfish motive, too. She'd been afraid for Merlin to see her own darker side.
Even as she was ashamed and worried of what Merlin would think, she was angry that Mordred would dare bring something like that up in front of Amy. She would rather Merlin think poorly of her than her young daughter. Amy's innocence was something Nimue would protect fiercely.
Between the conflicting emotions and all of the commotion, Nimue felt a bit faint. She didn't argue with the rescue workers when they told her they needed to take her to the hospital.