You accidentally walked in on Shinobu injecting poison in her body due to her strong bond with you she had with you she wasn't gonna tell you but had now too

You were speeding through the Infinity Castle trying to find Shinobu

It was too late when you opened the door where you heard Shinobu say "Go to hell" you find her being absorbed by Douma you knew you had to fight him until the poison took place

"Woah! Another one already? Just let me absorb her completely and I'll be back to you!"