Honestly, I would definitely confront her with your feelings about it. It doesn't sound like she has been honest or up front with you, and if you were under the impression that you were going to be exclusive or monogamous, then it is completely unfair and dishonest of her to call herself polyamorous and to date or mess around with other people.
If poly is something she wanted to explore with you, she should have brought it up to you and discussed it, rather than just having another relationship on the side behind your back.
She may not be ready or willing to have a monogamous relationship, and you need to be prepared for that if it happens. If poly isn't something you think you could ever be interested in, then don't do it just so you can stay with this girl. It can and most likely will lead to a lot of hurt feelings and resentment.