just found out my best friend is gay through twitter- why didn't she tell me?

[scruffy 14 year old]

me and my best friend (we are both girls, she is a year older) have known each otherfor 5 years, and i thought we knew pretty much everything about each other. i was just browsing through twitter earlier (which we both use, me considerably more than her) and noticed she had a lot of tweets about liking girls and retweets from gay/lesbian organisations and at one point she actually said 'i am gay'.
i had absolutely no idea. literally, i almost didn't believe it at first- i thought i was looking at the wrong person's page. we were talking about boys we liked at school just the other the day- does that mean she was lying? i would feel awful if i was making her feel awkward, but then again it cant be my fault if i didn't know. i just dont understand why she hasn't told me- looking back through her twitter she was talking about things like this for a really long time. i dont think she is worried as my other best friend (a boy) is openly gay and i have absolutely no problem with it in the slightest, and she knows that. i suppose maybe she just didn't want to tell me yet, but i'm not sure why. i don't know how to confront her about it, as maybe she doesnt want me to know yet. but, she is also making it pretty obvious on twitter so she obviously isn't embarrassed etc and she knows i look on the site regularly. what should i do?
@ Emma
what is?
guessing first deleted answer was a troll... ignore my first comment

I didn't think it was weird to look at what your friends were saying on twitter.
I was just wondering if I should talk to her about it.
(by the way- i have ASD (if you dont know what that is- look it up) so i find this situation very difficult)