Why is she avoiding me? what's wrong? It's just not the same.?

I'm 15 and I have this close friend.. these past days we become closer and closer everyday but today i think we're falling apart. the thing is she already knew I'm biual before we become close friends but I said I wouldn't like another girl again because that's just experimentation. Yesterday our other close friend was absent so that's why we are left with each other. she always ask me to sit beside her everytime the teacher is away and we talk and talk or just hang around. we both put our heads down and just stared at each other and play with our hands. it's kinda unlike her though, some people labeled us best friends and she replied silently "none of ur business, why do u care"? we're not bestfriends though just close friends. then this day comes and it's just not the same, we went to lunch with each other and then we hang around at this chair and she said "don't sit too close to me". then she kinda move her chair a little away from me and this happens again when we moved to another chair.. then after lunch she started being quiet towards me and not asking me to sit beside her like before. she also started hanging out this afternoon with this other friend more.. she's trying to avoid getting closer with me. then i told my other friends and her that I might not go to school tomorrow and she started staring at me not talking. she looks disappointed but I don't really know what's going. Any clue on what's happening? why is she avoiding me or scared that we become more closer? I don't know. any ideas? People don't know I'm or I have been one.. She's also very open about it. I tried to contact her at facebook but she won't reply like before. wtf was that?.. she seems to become snobbish too.. please help me?