If I'm supposed to be her best firend, why didn't she let me know about this?

[LOVE♥]

I found out through Facebook that my best friend is going to prom. She has already gone shopping and everything. I'm not sure when she got invited in all though. I guess she got invited to a group and they are having prom. I feel left out. Why didn't she tell me about it before? I mean like I had to find out through Facebook that she's going to prom. :\ She knows well that I'm not going to force myself into going to her prom. But considering that she tells most people that I'm her best friend, why wouldn't she tell me more about what is going on? She usually does. I mean like she doesn't come to me with her problems though and I've heard her complain about how the person runs away in the middle of a conversation. I guess I'm really touchy about it because I have an opportunity to go to prom at my school, but it's 2 hours away. So since I can't go to prom, I'd like to help my friend when she is going to prom. And plus the fact that she is my best friend, I'd just like to be there for her.
Why wouldn't she tell me before? And why doesn't she come to me with her problems instead of going to the person who runs away when they're talking?
*friend
She said it on Facebook, her mom commented and when I commented she replied saying that she got invited to the group/page where they are having a prom.
We're both cyber schooled at different online schools. I'm not exactly sure how she got invited, all I know is the fact that some group she's in was having a prom. She's only a freshman which is really odd since she's going to their prom.
I'm not interested in going to prom with her. I was just hurt that I wasn't told sooner that she is going.