<human>: Can you summarize the given text in a few sentences?

My [17M] girlfriend [17F] have been dating for 8 months, in which time we've had a decent number of arguments, due to a combination of me not thinking enough and her being somewhat emotional - she had a really bad relationship before this one which has affected her.

Well, yesterday we were sitting in my room and I was chatting to someone on Facebook about a party we were both thinking of going to, and she starts clicking on random conversations I've had. 

On one of these, she sees a joke my friend made a month ago about inviting someone I used to have a bit of a crush on (which amounted to nothing, and which my GF knew about already) to a movie marathon we were planning. I responded, jokingly, that I'd consider it. My friend had used that winky emoji, and I also said that "remarkably we are still talking" - I can see how it didn't look great to my GF.

Note that I'm still friends (although not particularly close) with this other girl, and see her around town/at the odd party - we've hung out since I started dating my GF.

At this point, my girlfriend gets up and storms home, saying that she can't trust me, saying that I shouldn't be talking about other girls like that etc. and she hasn't talked to me since.

I get that no one wants to see their BF/GF talking about a crush, even if it is a joke, but honestly it was nothing more than a joke, and one that was meant to be between me and my friend and nobody else.

What should I say to her when we do talk? How do I get her to understand that it really was just a joke?
<bot>: Friend made a joke to me about a crush I used to have, now girlfriend won't talk and says that I can't be trusted.