My girlfriend's kid simply hates me (long)?

You should get a video recorder and place it in a hidden spot in a room that you guys are always in. The living room or the kitchen would probably be a good spot. Be sure to hide it, but make sure the recorder can capture the whole room and the sound. Then, if something like this happens again, you can prove to your girlfriend that her son really is acting this way when she's not around. I hope this helped you!