"I've been going out with this" This!?!? Dude, you're talking about her like she's a piece of shit. But anyways, chances are she is the flirty type of girl. Start going up to her when she's with other guys and hugging her and kissing her so that the other guy's know that she's your's. Spend as much time as you can with her and flirt with her and make her feel wanted. A lot of the times when a girlfriend is flirting with other people it means that she feels unwanted. Make her feel wanted, give her everything. Try to go to some of the practice's and be with her so that she doesn't go to the other guys. If none of that works, then just dump her if it makes you that uncomfortable.