My girlfriend has been cheating on me but is calling it polyamory

My girlfriend has been hiding the fact she engages in poly relationships. I've come to find out after 4 years of our relationship my girlfriend enjoys polyamorous relationships. This caught me off guard due to her always saying the exact opposite. I don't know how I should feel, especially when I feel like she has been dating someone behind my back.

No, no, no, no, nope. That’s not polyamory, it’s cheating. If your girlfriend was dating other people “behind your back” or engaging in relationships that you didn’t know about, that’s cheating. Full stop, end of sentence. I am sorry that she is mis-using the concept of polyamory to try and paper over the fact that she violated your trust and the terms of your relationship. It’s not that you “feel like” she was cheating, lying, and hiding things from you - that’s an observation of reality, not a feeling. You are justified in feeling hurt, angry, betrayed, violated, and whatever else is coming up for you. It is probably time to end this relationship. I am sorry.