I have something similar. Very short outbursts of violence (punching something, throwing or kicking something, shouting) which can be followed minutes later by crying, withdrawal, fear, anxiety. While, mine seems to be intensified by starting new medication, I spoke to my therapist briefly about it. She thought that it may be a reaction to bottling up emotions, because if you bottle up your feelings they have to come out somehow.

My anger is usually aimed at a wall or a floor, but in the past (different manifestation but same idea) it would be directed at people I had anger or frustration towards but never said anything. Is there anything bothering you that you haven't discussed with your girlfriend? Maybe taking some time for reflection... and also trying to recall what you were thinking about before you have a "rage", even if it was hours before, may help! Maybe keep a journal of these thoughts for when you do see someone.