Why is my ex boyfriend doing these things?!?

I am not sure what your ex is trying to achieve here - all it seems is that he is trying to mess with your mind. Flirting with you whilst he is dating another girl is not the right thing to do, and the way he uses his girlfriend to make you jealous seems like he is using her to get at you. This guy does not seem like a good guy. The best thing to do would be too stop contact with this guy - a clean break will help you to move on with your life and not get too worried about what this guy is doing