<human>: Condense the text into a few key points

My girlfriend has a male friend who she regards as kind of hers BFF.
I thought nothing of it until last weekend she was in a bad mood over something. I asked her what happened and she told me that she got jealous over her BFF who is spending time with our common female friend. In fact, she was so jealous, she was offended and took it off on him.

I am kind of an insecure person and considering that she met this friend of hers after we started dating, I'm starting to get a bit worried. I trust her. But I also know that she is inexperienced and emotional person and, frankly, for a long time already I think that our relationship is based on some kind of mistake. Like we got together because we just both were lonely and willing, and there was nobody else available.  I'm starting to think that her condition has changed. Mine hasn't, though. I still love her but I'm not sure that it's mutual anymore.
<bot>: I suspect my girlfriend started to develop feelings for another man, but I'm not sure that it isn't my insecurity speaking.