She don't trust Tybalt, but if you and Jill aren't a couple, she has every right to doubt you.
Think about it: she doesn't know you that well in this case, yes you had a date, you drank wine with Bella and you helped her at the concert (optional)... and that's it. But she knows about you: the fights, you ran around campus naked, she knows from Bella that you get into trouble, you snuck into her room (breach of privacy), you crashed the preps party. Her doubts are logical. She doesn't trust Tybalt, but she doesn't trust MC completely either, you don't have the connection to calm her doubts.