Spears, 24, had requested the interview, broadcast Thursday, to counter recent criticism of her appearance, relationships and parenting, but told Lauer, "I do not care what people think."
When asked why public opinion of her had changed, she guessed it might have been the breakup with Justin Timberlake, who had accused her of cheating.
Spears said she had not known Shar Jackson was pregnant with Federline's second child at the time Spears and he struck up a relationship.
"I didn't know until two months later," Spears said. "But I don't blame him... They weren't technically together when he came to me anyways."
She insisted Federline is very caring. "He has a really big heart, and I love that," she said.