I love how some of you think human emotions are black and white. Yes, she understand, but it does not mean she is not disappointed that something she wanted didn't go the way she wanted. You can't judge her for this.
She like a guy, and that guy chose someone else over her. She would be more understandable if its actually a relative to MC , his father his mother , sister, brother. But in the end of the day its just an another bitch , and Jill at this point knew MC dating with others. And Sage is not dying, its just having a fucking flu.
so yeah Jill is selfish, so what? No One actually bother to see this from Jill perspective.
I get it that it might be a sudden shift in Jill personality, but its miles better, than her sudden shift, of believing Tybalt based on just his words.