>>279965798
She's incredibly embarrassed by it and feels like she needs to shoulder the burden of it herself, as if it's punishment for her own existence. It's pretty understandable she doesn't want to tell them the truth.
The less understandable part is how hostile she gets about it. I could see it as her trying to cover up for why she's doing this so none of them try prying deeper. It's also a pretty big part of Ave Mujica's story that Sakiko ignores the feelings of others when she's focused on something so I guess it makes sense.