She's not talking to me anymore, is she waiting for my reaction?

Hey sedditers! (hope this is the right term, since i just discovered this subreddit this morning...) first of all, I have no experience at all with seduction, i'm a 18 male, college freshman. I've been "dating" using only my shitty intuition... I'm a virgin too. Hope this information makes easier to answer my question...

So, i've been dating this girl for 2 months, she's 15 so.. yep, other redditors have told me before to NOT date girls this age.. but I want to ignore that advice for now. Everything was great, we had great times during these two months. Two weeks ago, we spent the whole weekend together at a friend of hers house.. too bad the girl i was dating before her was there too, and they're supossedly best friends...

So, after that weekend, she started to act a little bit weird... eventually not paying me attention. We usually talked through facebook, everysingle day, even if we had seen each other that day. The last time i saw her was last sunday, we had a little fight because apparently i'm rude to her friends and she mentioned she believes i still like her friend which i dated before her... she had to leave before we finished talking. So, i arrived home and wrote her a private message on fb. Since that, i haven't know anything from her, she doesn't reply my sms, neither my fb messages and i know she HAS been on fb all the week because she's been uploading shit.

**TL;DR: a girl ive been dating said she believes i still like a friend of hers ive been dating before her, and is not answering my messages since sunday. I showed some weakness by meantioning i didnt want to lose her. **

What should i do? should i go to her place and try to talk? move on? try to figure out what's wrong? what is she trying to prove or what's her objective by doing this?

THANKS! Sorry for my perhaps bad english, i'm mexican, so, yeah.