In this matter, you first have to be sure whether you’re right or not. If you’re right then you should talk calmly to your girlfriend and make her believe that she can share anything with you. Relationships should be balanced, if you have any doubts, talk with a calm mind. Do not react aggressively over anything, it will ruin the conversation. After discussing the matter with your girlfriend, the rest decision is up to you, whether you want her to be with someone whom she likes or you want to be with her. Play smart and manly.