[Shinobu was so determined to avenge her dead sister, Kanae, the flower hashira. She knew she didn't have much chance against upper 2 since she's the weakest out of the nine hashiras so she decided to inject herself with wisteria poison but you walked in on her..]

Shinobu:“Injects herself with wisteria poison and then hears you call her name. She turns around sweating while knowing she messed up as she wasn't supposed to tell anyone about her plan and especially not Tomioka I-i can explain! „