I don't know what's happening

So I’ve been with my girlfriend for a tad over 6 months now, and I’m at a complete loss about what’s going on right now. We’ve been really close friends since early college days, and we shared just about everything. We’ve even seen each other fail in past relationships, supporting each other along the way.
But here we are, ended up together in a serious relationship, and 6 months in already. Things were great, and things seemed to be going well. We recently planned a trip to Vegas together, and all was well. However, this past two days just turned into a snowballing pile of turd that’s raining on our relationship.

I have a female friend from school who I chat with every now and then (along with the rest of our research group). One day when my gf and I were just laying in bed and chatting and watching a youtube vid on my phone, my friend messages me. The phone displays the message (in shrunken size) on the bottom corner of the screen to let me know what she said. So my friend tells me she just woke up from a nap and she’s energized and ready to study (finals are coming up). A very innocent few lines, and my gf catches a glimpse of the messages. All of a sudden, it’s as if she flipped a switch. She suddenly said “let’s not go to Vegas” and started to tear up. Then put on her jacket, and proceeded to get ready to leave. I tried to get her to talk, but she’s never been the type to talk when there’s a problem (which doesn’t help whatsoever). So we sit in silence for something like an hour, where I’m basically staring at my gf crying while I try to ask “what’s going on?” Frustrated beyond imagination, I just let her leave since she won’t speak a word to me.

Later that night, she said she’s bothered by my friend texting me so much. Mind you, I never responded to any texts while we were spending time together that night. She proceeded to say that if I didn’t text my friend back normally, my friend wouldn’t be texting me so much randomly. She also says that my friend sent a flirtatious text to me by telling me she woke up from a nap. For the record, my friend (along with my whole research group) knows that I’m in a committed relationship.

Just about an hour ago, my gf sent me a text saying “you knew me better when you were my best friend.”

I am totally lost right now and I feel kinda confused. I don’t know what to say, and everything I’m saying seems to be somehow flipped into something against me.

Comment if you’d like, but I just wanted to post something to vent my frustration.