Have I Lost My Best Friend?

Ever since I was, maybe, 5, I've known this girl. Throughout our lives we were those two inseparable kids, it was never a relationship or anything, we were always just best friends. I'm 23 now, she's about two years older, and we hadn't seen each other since the end of my senior year of college. Since I had graduated and gotten my own place, we started texting some plans, things got arranged, it was decided she would crash at my place for a few days while she drove back to meet with some family about a state over.
So the day comes, I'm really excited, she comes to the door, and It turns out she's been taking hormone treatments and stuff to become a f->m transsexual. This wasn't guess-work stuff it showed. Some spotty hair, she was dressed in some super masculine clothes, her chest -- really trying not to sound pervy here -- was totally flat. I was really surprised, she never seemed the type to be uncomfortable with who she was, but she was my friend so I really didn't say much.
So we get to some small talk, I try to avoid the topic that's bugging me, and she decides to bring it up to 'clear the air'. Apparently since about high school she's felt this way, and she felt finishing college was the time to finally transfer over. She finally gets through her story, and I ask about how she wants to be addressed and stuff. She goes by Max instead of Catherine now, and she really stressed the whole "he" thing. I still didn't feel very comfortable with it all, knowing someone your whole life and then seeing them completely and utterly different just shocks you, I felt sort of hollow, I don't know why. We managed ok for the next two days, the tension slowly dissipated as I got more used to her new look, but one day I accidentally called her Catty -- it was my old nickname for her -- and she snapped at me.
Even with the whole anonymity thing, I don't feel comfortable with the whole speech, but, she was angrier than I've ever seen her before. I guess that's when it sunk in how much things had changed. I tried to apologize, but she was just really angry, I might've slipped and called her a girl too, I don't know, I was just worried and scared, and then she just took her bag and left. I thought about following her, but I just was too stunned, I didn't know what to do. It's been a few hours, I just googled anonymous story to get here, I dont know who, I dont know from where, just, someone.

What should I do? She's my BEST friend, but I just feel lost.

EDIT i worked up the courage to call him. he was calmer, and i was able to apologize and patch things up. We're ok ish right now, hes already mostly to his family, so we'll hang out when hes headed back.
GaFi GaFi
22-25
Dec 2, 2012