What Causes Anger After Starting Birth Control Pills?

HI, my nam is nick, i am ver concerned with with my girlfriends health. she has started on birth control pills 2 day ago. ever sense she took them she has been different. she is not the sweet person she was before. she has never been angry with me before but yet today she has been angry with me 2 times. i do not know what to do and i feel so bad. i tell her to stop taking them but she will not listen