Anna's smart. She is a med student, after all.
And, she's a smart enough girl to realise that Hans pledged himself to Elsa but didn't keep that promise. His own promises, whispered in the heat of the moment, probably mean even less. He'd been more than happy to soil her, use her – she'd wanted to use condoms! He'd been desperate to get her on the pill, completely abandoning latex once she was.
She's told him it was Elsa's, and his only reply was, "But I love you, Baby."
It made Anna sick.