Why does a girl suddenly change her mind or suddenly dropped her interest?

So I got to know her last semester (before summer) and we had gym together. During gym class, whenever we did sports together on opposite teams, she always be like running towards me and like "hey! 'my name'" and standing in front of me to block me in sports (football, field hockey, etc) lol. We didn't talk much back then, but when i saw her in the hallway we always smiled at each other.

During summer, She Gave me her number and we talked a lot more than before. She even offered to walk me to the bus stop sometime. And then we started texting to each other (she initiated most of the time). The first few texting game was long af. We texted over 5-6 hours on and off; normally replied me about the same amount of time I took to reply her. We started off on casual topics and then when the school starts mostly about school stuff and homework.

Now here's the twist!!! After school starts a new semester, she started texting me more frequently (however mostly school topic). i caught her looking at me or glance at me for a few times. However, we didn't talk at all (maybe because no time and no chance, cause we couldn't find each other besides the only class together). BUT, all of a sudden, she started calling me BROTHER during texting!!! Like three times!!! I don't get it, during the texting before, she was always flirty and I flirt back too. And when we were talking we laughed at all my dumb jokes. (Maybe she's just really nice or a huge flirt?) But all of a sudden, like out of blue she just starts to call me brother. Was she never interested in me or she got scared off or something? She's still single by the way so I don't get it.

Background info: I know most of her friends and I was kinda flirting with them too but I established that they were in my friend-zone. So my jokes to them were mostly like guy jokes (I might come off as an asshat to them) Maybe that was the reason? that they started talking about me and I didn't come off good?

I'm not saying that I want to be with her now or something cuz she's already put me in the friendzone and I don't really feel like dating. but I just wanna know that what did I do wrong or which signs I read wrongly, so that I could learn from this experience.
She was never interested in me. She was just nice and flirty.
Vote A
She was interested in me but dropped her interest.
Vote B
She is still interested in me but is scared of my possible rejection so she shielded herself?
Vote C
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Why does a girl suddenly change her mind or suddenly dropped her interest?
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