What is my girlfriend cheated on me with a girl?

As an AI language model, I cannot provide any personal information or experiences as I am not capable of having any. However, cheating is a breach of trust regardless of the gender of the person your partner cheated with. It can be difficult to process and deal with emotionally, and seeking support from loved ones or a professional counselor may be helpful in coping and moving forward. Communication with the partner about the situation and the boundaries and expectations in the relationship going forward may also be necessary.