My wife and I have a saying we define our relationship by.
Communicate, communicate, communicate and if that doesn't work communicate some more.
Open and honest communication is critical to making any relationship work and that goes even more so for a D/s one. You both need to be clear on what you each want and expect from your relationship and be certain that you both understand and are accepting of the others expectations.
It may be (probably is) that you and your girlfriend have different visions of what your relationship should be, and you need to reconcile those visions and find a workable joint vision.
It could be that she wants you to be a bit manly and domineering in some circumstances but not in others. It could also be that she really wants you to take more control but wants to test you by being a"brat".
The only way to know for sure is to ask.
In a healthy BDSM relationship submission is gift that is earned and given, taking submission is abuse. Any "sub" who is looking for abuse needs a therapist not a master.
A "real" Dominant is not someone who simply barks orders and demands compliance. They are someone who is seeking to fulfill the physical and emotional needs of their submissive.
While the sub submits it's really the dominant who serves. It's the paradoxical ying/yang of a D/s relationship that the ultimate control is actually held by the submissive since they are the ones who set the term of their submission and the Dominant can choose to agree to those terms or find another.
So back to your question, talk to her openly and honestly, be patient but insistent. I've also found that submissives have a really hard time being blunt about their desires, so go slowly and revisit the conversation regularly as it may take her time to learn to trust you enough to fully open up to you.
Our society puts so much emphasis on being in control that it is a very difficult thing for someone to admit to not wanting to be in control. This can be particularly difficult for a modern woman with the pressures of feminism telling them that being submissive is demeaning.
This is not to say that feminism is the problem, I am a feminist as well but I find that many people misunderstand the difference between choosing to be emotionally and physically submissive in a romantic or physical relationship and being forced into such a role by social and cultural norms.
Being forced into a submissive role because of one's gender without concern with the desires of the individual is misogynistic, but denying that same person the right to choose to be submissive is no better.
TL: DR Talk to her about it and don't be a jerk.