So today after school one of my best friends walked up to me and said that she wanted to talk to me about something serious. So I asked her what and she said promise not to get mad at me "I don't want to be friends anymore". I have no idea why she said it. Everything had been perfectly fine between the two of us until she uttered those words. I asked what made her feel like that and she said it was personal. I don't know what I'm supposed to do with that, she was the first person I ever trusted to be my friend but now I'm doubting all of my friendships. I don't know what I'm supposed to do at this point.

Either confront her about it or let it go.
Or alternatively you can just wait for a few weeks pondering all the possible reasons and then finally ask her.

It's difficult, but sometimes you don't ever get to find out why someone you considered a good friend leaves you. My best friend of 10 years just suddenly stopped talking and replying to me, had to let it go.
Give it some time, maybe your friend has some problems she needs to deal with. Then try asking her again. If it doesn't work, you're probably better off just letting her go, even though it sucks.