When you got aggressive towards the guy she was talking too she saw a weakness and an insecurity in you. Girls get turned off by that. It reaks neediness. Your best bet now is to take a step back and slowly reinitiate contact, because you've made her defensive towards you with your lack of understanding attitude. Lucky for you you can change that. In the future when a guy tries talking to your girl, you can do 2 things
#1 keep an eye on them, if is just a casual friendly conversation then let it happen(ex. make sure he is at a distance, his hands are off, and he doesnt look like he is flirting, also if she doesnt seem into him)
#2 If he is obviously trying to flirt and it looks like he might be a potential cockblock, immediately step in and just put your arm around her or just drag her towards you while you smile and just let it be known she is yours. (Is all about your innergame here,the frame your coming from is "She is yours and you wont accept another guy takin her") Is all about her, you never have to address or confront the guy. Remember you are just being social and girls wanna have fun, get her attention back to the real winner(which is you) and just be cool with the dude, but do not allow him to make moves on your girl, simply drag her away or just show that shes with you. But every move you do is on the girl, never the guy. Stay cool and be friendly, show the women a good time and she will respect your desire for her.
Good luck.
Here's a video example of being dominant and leading
https://www.youtube.com/watch?v=b3wWlkXCKqo
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