Why is my mom convinced that I fought over a guy?

The truth is it wasn't over a guy but my boyfriend's ex came to our neighborhood's cook out uninvited and started flirting with him so I asked her not to flirt with my boyfriend and told her that they were over and she needed to move on. Then she said something like she was better then me anyway and he wanted her back so I laughed in her face and told her that would never happen since he dumped her because she cheated on him with a couple of guys (I know this for a fact since we were friends at the time, it's a long story I'm not going to get into right now) and because he liked me better anyway. Then I told her that maybe she needed to be tested because who knew how many STDs she had so she slapped me hard! So I had to hit her back and we started fighting until it was broken up and my mom sent me home and later tonight she told me how I embarrassed her and how it was "unladylike" to fight over guys but it wasn't over a guy, I would have never hit her if she hadn't of slapped me! But of course my mom is going to believe what she wants and she told me that I'm grounded until I learn how to act in public so I can't go anywhere but soccer practice and back home now and I can't even have my cell phone or Ipod either! But what I don't get is why she is so convinced it was over a guy when it wasn't! I know she still thinks that and that she's going around telling people that as well. What is her problem? Why would she think something so stupid? I mean I will never fight over some guy!