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(eg hair flicking, leaning in, laughing alot at whatever it was he was saying etc)
Okay sometimes read into things a little too far. These are all things that girls do even when they aren't flirting so you cannot base you argument on these things. Girls flick their hair to get it out of their face. Girls (and guys) lean in when they can't hear someone, and girls laugh when something is truly funny (go figure). So before you get all stressed about this take a step back and realize that she may not have been flirting hardcore like you thought.
Now, that said, there is a chance that all those things weren't just a coincidence and she was flirting. In that case you need to step up and be a man. If you just sit back while a guy is flirting with your girl and you get pushed out of the conversation, that just looks bad. It looks like you have no backbone and people can step all over you. You don't need to tell him to get lost (unless he is really inappropriate) because that will make you look jealous and secure. But you need to send a message that she is your girlfriend and you can't be pushed around.