This woman is the first person she's met who hasn't immediately plastered on a fake smile and said that she's fine. That delay before she responds that makes Lena believe her right away. She's struggling with something but it won't break her, and that's enough for Lena.

Lena smiles warmly in response.

"'course. D'you wanna talk about it?"

She won't be offended if Diana doesn't take her up on the offer, but she may as well make it anyway.