What if you woke up tomorrow and you were a boy? Everyone argued with you if you said you were a girl; they made you use the boy's bathroom and told you "boys don't cry" and so on. Would that bother you?
Serious question - I don't have an internal sense of gender myself. I wouldn't care if that happened, aside from being a little confused that it happened overnight. But I had to accept, after watching how much my wife struggled with it, that gender mattered a lot to *her.* That's really all that matters; you don't have to understand how your sister knows she's a girl or why she cares, just that you love her and want her to be happy, and this matters so much to her. In the meantime, just telling her you love her and DO support her in what she needs to be happy might help. I can totally understand why you want an explanation from her, but obviously she's very sensitive about the subject right now, and that's not going well. Maybe, in time, as you show that you still care and she becomes more comfortable with herself, she'll be more able to talk about it.
I do sympathize, though. It took a long time for me to wrap my mind around the idea that anyone cared that much about gender (when I was much younger, I assumed it was one of those silly and pointless things people insist on making everyone do, like small talk!). I'm assuming that if she's older and she just went to college, you're under 18 yourself, which also means you've got a ton of other stuff to worry about right now.

It's no wonder that this is throwing you for a loop, when just surviving middle/high school is tough enough by itself.