My best friend going out with the guy i like should i be mad?

k soo ive liked this guy since september when i started my new school nd about a week later my best friend announced she liked him too.i didnt think about it that much coz i never belived anythin wud ever happen.la few days agoo i found out that there goin out.i felt soo sad wen she told me nd wudnt speak to her fer the rest of the day and she never said anythin more about it.i had never felt soo hurt by her in all my life -nd if u read my other question youll see what else she been up to nd why this upset me soo much-i wanted to scream at her nd jst lie on the floor nd cry.she really didnt care atall bou my feelings nd just went ahead nd did it buh she knew how much i liked him?the next day SHE was the one ignorin ME.and in very class i had to sit beside her wile she over exageratedly luaghed with him-even wen we had PE nd we both werent doin it we sat beside each other on the bench alri but facin the other way wile she listened to her ipod nd ignored me.im nt sayin that like she had to ask my permission to go out with him atall!buh with the way shes actin soo cool bou it makes me feel like im the one whos over reacting?somethin similiar happened last yr too.i had like a guy fer like a year nd she told everyone she liked him nd everyday she wud be all flirty with him nd i truly belived he liked her bak so i never said anythin bou it,baked off nd lost interest in him.tell me the truth am i over reacting?i really belive i am right now.she hasnt spoken to me today either so why is she the one with da problem now?plz help!