I want to tell my gf she's a bitch.
Why would you say a derogatory term to your girlfriend?
I was mad at her and I wanted to hurt her feelings
Calling your gf with that word is a bad idea. Just be calm and wait it out or tell her how you are feeling.
When I'm mad, I just want to hurt her feelings.
Hurting her won't solve your problem. You need to communicate and tell her why you are upset. Calling her names might make her angry and vindictive.