Is my girlfriend ignoring me or could she still be upset?

She's probably still upset. Upset is pretty much every teenage girl's default setting, along with neurotic and emotionally unstable.

Oh sure, call me a sexist bastrd for saying that. But do you really think I just made it up out of thin air? Hell no. Teenage girls have a reputation for being kind of completely insane, and it's a reputation they earned over thousands of years.

Give her a day or two to cool down, and everything will be fine. Well, everything will be fine until her next emotional meltdown, at least.

The best apologies are in person. Your first mistake was letting her walk away because now you dont know where she is and whats shes doing. The next time you see her, in a serious tone you say everything that you love about her and all of your own flaws and you DONT let go of her. The most effective apology I have done is this but I managed to shed tears to have her take me more seriously. Dont be dramatic, you're trying to win her back, recalling one the first memories you had of her could work. The fact that you're blunt is a good tool to work with, use it so it looks like you arent a liar.
Lastly, dont forget to say I love you because at the end of the day, she will remember those words the most.

leave her alone, give it a day or two. then apologize and bring some little present