You should have stated explicitely that you were a girl, though I guess the "another girl" should've given it away.
Well... I can't really speak for you, because I'm not in your head and I'm not a girl, but I guess that if you considered her as a close friend and she told you she sees you as casual, it's a bit like being rejected romantically. When you think of someone as a "best friend", you're creating a bond of sorts between the two of you; the other person telling you you're a casual friend is denying you this bond.
So I guess it's normal you've been pissed off and affected deeply by this; the fact you 'changed' a bit (the emotional outbursts), be it subconsciously or not, might reflect the fact your "pride" has been hurt, and you wanted to correct the fact that "you weren't a very open person".
I think it's not a big deal (the outbursts); although it may seem annoying (and it can become, don't let this go too often), it shows a proof of your character. You know, if you keep everything inside, people get bored... even if you do it because it's logical to do so if you can.
Then... you might have developed romantic feelings for her, i really don't know. I'll wait for part 2 if there is one.