Why would she mention "sitting down" all in sudden? Is it some kind of an idiom?
Native speaker’s answer
No, this isn't an idiom! "Sitting down" refers to being in a sitting position on a surface. But it does have a kind of meaning behind it in certain contexts. When someone has big news to share, or something difficult to process, they often say, "sit down" or "are you sitting down?" to indicate that it could be too much to handle emotionally, perhaps with the possibility of fainting. Ex: It'd be best to sit down before I tell you the news. Ex: Are you sitting down? Okay. I'm going to win an award!