First off dude, since she’s talking to her ex, she needs to think that you are willing to walk away from the relationship at any minute. Girls think that they can talk/flirt with any guy, if they got their boyfriends balls in a vice grip.
For example my last girlfriend and I were watching a movie at her place once, and when her ex called, as soon as she picked up, I pushed her off me, got up and walked to my car. She hung up and ran after me, grabbing my arm and beged me to come back inside.
You're probably off your game, because you stopped talking to other girls, since you have a gf and use that as an excuse not to go out and sarge...bad idea and a lot of us do it. Going out sarging while in a relationship keeps you sharp and on your game. When your gf sees that you have other options, if you so choose to dump her, two things will happen. 1 her jealous side will trigger. 2 Your dhv will be at an all time high making her attracted to you.
My buddy is living with his gf, and every day she sees him skyping/facebooking/IM 10s of average to HB girls. She knows hes not cheating, but at the same time she’s in constant state of mind that she could lose him. It seems messed up I know, but if he didn’t keep his friendship circle of HB, she wouldn’t be as attracted/devoted to him and at any point could just walk away.