<human>: My girlfriend and I have been together just shy of 6 months.

e're trying to go to a music festival this summer, but after looking how expensive flights were we were thinking a bus might be better. I was talking to a close friend of mine (girl) and she said they were driving out from the same city the bus leaves and still had space in her car. 

Now this girl is my best friend's really close friend. I've only met her in person for like a week during which my friend set us up, but since then we've cleared that we're over that and we're better off as friends. We talk often, but I feel I should mention we don't talk like "that." It's really just how you'd talk to a friend, no flirting from either of us.

My girlfriend asked me the other night if she was going to meet any of my past exes/hook-ups during the music festival and I told her truthfully  about this girl. She blew up at me because she's noticed me talking to this girl throughout our relationship and thought I was hiding it from her.

I just didn't feel like it was need to know information to tell my girlfriend about her. I didn't think they'd ever meet each other, and so when this carpool situation came up I knew I would have to. I keep telling her I would've told her, but she keeps twisting it like I was going to tell her last minute. I've known that we were going to ride up with them for all of a day, and didn't want to bring it up then because she was upset about something else. 

 How do I explain to her that we're really JUST friends? She doesn't believe me that I would have told her, and says she'd feel like an idiot driving up with them where everyone knew except her. She says she can't look at me the same way anymore. How do I fix this?

Can you condense the text into a few key points?
<bot>: Gf found out I used to hook up with one of my close friends and thinks I've been hiding it from her the entire relationship. What do?