My Girlfriend is talking to her Ex? HELP (Lesbian)?

[Teen Facts]

Hi , im 16 and my Gf's 15 we've only been dating for like a month ,but we really like eachother. My girlfriend was talking to her ex telling her she loved her and crap, her ex sent me screen shots and everything i broke up with her but got back together (i just really missed her) she said sorry and we moved on but on super bowl saturday here EX came OVER HERE HOUSE WHILE I WAS OUT OF TOWN. This by far pissed me off , i felt disrespected and played, got drunk that night and just blah it was all bad. I really care about my girlfriend and she says she cares about me but i dont really see it. She told me she'll do anything to gwt me back so i said never talk to her again but turns out she was b.c they saw eachothet for the superbowl when she went to her house. Her ex is in college in Dc but she comes to visit. I just feel like im being uses as a pawn her ex keeps talking about how she can get my girl friend and crap (on twitter) shes 18 but shes very immature.