Can anyone tell me what's going on?

I have a best friend. and she is so awesome and fun, and you know she is a great person. but she makes me mad very easily..and i'm not an angry person. like when she constantly talks, and talks. and like when i'm ina bad mood one day she gets all mad cuz i'm not talking or i get kinda mad. but when she is ina bad mood i don't get all pissy with her. when she is ina bad mood she keeps to herself but she can act like a B**** also. and yes i know i can get mean when i'm ina bad mood, but i know that. and i think she would at least understand that when i'm not having a good day. and i understand when she is having a bad day, that's why i don't get mad at her...but she get's mad at me when i'm ina bad mood...it's stupid. I don't want to lose her as my best friend cuz we aren't in bad moods alot but when i am it's annoying when she gets mad at me, and i'm getting sick of it...Please i need some advice...Thanks!