Feelings are natural, and it's okay to have emotional reactions to situations. However, it's essential to understand the context and communicate openly with your girlfriend about your feelings. Being angry solely because her friend gave her a matching necklace might not be entirely reasonable unless there are specific underlying issues that concern you.
It's important to remember that friendships can involve giving gifts as a gesture of affection and closeness without necessarily implying anything romantic. If your girlfriend and her friend have a close and platonic relationship, there might not be any cause for concern.
Instead of letting anger consume you, try talking to your girlfriend calmly and express how you feel without blaming or accusing her. Open communication is crucial in any relationship, and it will help you both understand each other's perspectives better. Ask her about the gift's significance and what her friend meant by giving it. It's possible that the intention was innocent and friendly.
Keep in mind that trust is a vital aspect of a healthy relationship, so if you trust your girlfriend and have no reason to doubt her loyalty, it's best to give her the benefit of the doubt. If, however, there are other concerns or behaviors that are contributing to your feelings of anger or insecurity, it might be worth exploring those issues together as a couple.
Remember, each relationship is unique, and it's crucial to establish boundaries and expectations with your partner. But always approach these discussions with respect, understanding, and empathy.