Are we splitting up? PLEASE ANSWER?

Ok fellers, so this is what's happening. My best friend and I have been best friends since January-ish, and since then we hooked like the snap of a finger. We like the same music, same friends, same food. All of that. So school ends, and over the summer I'm busy workin on the ranch, feedin dogs, hunting, all that. But we still talked. But we didn't hang out much. So this year of school comes around in August, and we have math together. we sit next to each other, and I make her laugh everyday. Then mid-september rolls around, and she starts acting real strange.She started actin like one of them spoiled California rich fellers. Last year, she was good-lookin, but not wow. I was still nice to her, even though my friends said she was "un-cool." Then over summer, she became beautiful. Braces came off, her grew ALOT, and she lost. I compliment her a bit more, and she blushes. But now, I'll try to say something nice to her, and she goes pssh ok thanks, like she doesn't appreciate it. I didn't over-compliment her, and she's purty popular now. I mean i don't wanna brag, but I'm popular at my school, everyone knows me, im the all around nice guy, and people love me. I think she just used me to get popular. Before, we would talks for hours, and now, we can't have a five minute conversation without her pissing me off by being short and showing attitude. She became real guy friendly, hanging around guys since summer all the time. Maybe she thinks I'm just another dog after her pants? Nope. i actually care about her, and why would I want to get in her pants right NOW, I'm only 14 0_0 but i can't tell if it's my fault or hers. What do y'all think? And if you read this, mucho gracias amigos y amigas ;)