Why is my best friend like this...?

we ar 21..lets say her name is sally....she is my good friend since 2 1/2 years...and i dont hav many friends too..she is my only best friend and we used to share many things like shopping,restaurant,sitting together in classesetc..she lives in hotel and im a daystudent...there is another girl lets say her name is cathy she is my next roll number girl...and we shud do some things together like projects etc.,so the problem is sally doesnt like cathy...dont know why really nd i never dared to ask her(i felt there is no use of asking her)..and when i talk a little close to cathy or hang out with her..my best friend try to ignore me whn she is around ..like when me and sally sit by the side of cathy in class...nd whn i get to conversation with cathy...this sally girl behaves like im no where there...this happend before many times nd once i got irritated nd stopped talking with her for like two months..this is 5 months back....and she ignored me like hell....in those two months when ever me and cathy cross her in college corridors she used to keep that constipated face and did many **** things.the good thing abt her is she kept my secrets during this period and didnt talk sht abt me(as far as i knw).. and later she called me on fone after two months...and we are like best frinds before.... but she didnt talked with cathy....for this day too....and now the problem started again....my best frnd doest talk with me freely whn i make conversation with cathy she is around.....nd as both sally nd cathy are hostlers....sally is hurting cathy with her looks and etc in hostel....what shud i do...its annoying me and irritating me....and i dont feel like talking with sally...i mean a person can hav many frnds right...and sally also lost one or two frnds before like this...i asked her whats the reason...and she told that i expect the same importance i gave them from them....(they used to talk with another girl with whom sally broke up with)...shud i move on and make new frnds or shud i stick with this girl...ur opinions are very help full....sory for the long length of my question and thanks for sparing ur time