You and Mandy are both interested in Lip Gallagher's well being - for different reasons, of course, she's been with him for a while and you're simply an old friend that never considered even the faintest idea of hooking up with the seemingly irresistible lad.
Even if she doesn't trust you completely, it's a fact that you both have an enemy in common: Karen Jackson. Not only she gave you real reasons to hate her, but she was still meddling in Lip's life to this day, trying to sabotage what seems to be far more healthy with Mandy. The rest is history. You helped her hitting Karen with the car and now you both carry this guilt within yourselves. Unnecessary to say that Lip is mad at both of you.
"It's you fault, you said it would work!" she was still itchy about it. The girl's in the hospital and Lip doesn't leave her side, great. Not only it went wrong, but now you have to offer some shelter to a Milkovich because she has nowhere else to go.
"And you know what? I think this was just a plan! You were dying for him to hate me so you could have your way in, right?"
Wrong. You actually didn't care about having that type of attention from Lip because you simply weren't interested in boys. No one knew that tiny detail about you except for him, but now that she's getting so annoying, you kind of think about telling her about this. So infuriating.