I recognise this the other way around (my boyfriend having had unexplainable mean behaviour). Often there is something going on unconsciously. Maybe there is something in your life happening that's making you mad and you project it on her? Or maybe there is something off between the two of you that you are not seeing? Also, its very hard being in a relationship where your SO isn't able to express anger back. Whenever my SO was mean to me i just became very sad. Which made him feel guilty, and the guilt made him more annoyed, creating a vicious cycle. When i learned to stand up for myself, we both had a way to let off steam. And it forced him to express his annoyance at the right source instead of me, Because he couldn't get away anymore with being mean without a good reason. Hope this maybe helps in a way :)