my friend always acts like she's all tough? yesterday she kicked my ALREADY INJURED KNEE??

So my friend (15) used to be in karate. Yeah, big deal, right?

Well, whenever we're messing around, she starts saying stuff like "you better watch out, I was in karate!" And I always reply that just because she was in karate that doesn't make her any more tough than I am. I still work out and lift weights.

Yeah, so she always acts all tough just because she was in karate. Um, chill, you're not the only one. I know friends who were in karate and I never even find out until I ask them what sports they've done. As for her, she just brags about it without me even asking her. "So I was in school, and this kid was annoying me, so I threatened him and told him I had a black belt." "Once this kid hit me for fun, so I kicked him and he backed off." He didn't back off because he was scared. But he's not gonna fight a girl.

Yesterday we were at the swimming pool and I pushed her into the water. So you know what she did? She kicked my knee, which I injured a few months ago and is still healing. When she kicked it I screamed. It KILLED. I started swearing under my breath and at her, telling her I've been waiting months for my knee to heal and then she just kicks it. She started apologizing, saying she forgot that my knee is injured. Well, now I'm scared that it got worse and I wanna get an MRI done. And it hurts.

My brother said it's my fault that she's in the position where she does these things, and that I need to let her know not to touch me anymore, even as a joke.

Thoughts?