Why have I been so bitter lately ?

For the past year .. I have been so bitter .. to my friends and I really don't want to but I use to be so nice but lately I've been mean always thinking that people don't like me s: .. I've just turned into a bitter and angry person. I can't stand it .. s: .. I use to be so nice.. never fighting with my friends , never getting mad at them for some random *** reason.. (Not only on my period days so dont say Im pmsing cause I am deff. not) . I feel so down and agrivated . I dont want to be like this .. I want my old nice personality but lately the bitterness is just taking over me.. Its so stressing to fight all the time..