Your boss either likes/loathes you, or into your girlfriend/using your girlfriend to manipulateyou, and is in a backdoor way trying to hint to something. Ask him man to man, if he has a real problem with your work performance, and to not use your girlfriend as a tool to undermine you.
At the very least he knows that you have the stones to ask what is up, and that you know the game. This will garner some sort of respect and level the playing field. At the very worst, he'll continue down the same road with more games or will fire you.