Welcome from a former Michigander.
I would suggest don't make any big decisions until you've learned more about polyamory: both in general, and in terms of how it's practiced by your girlfriend and her other boyfriend. Even if you ultimately decide to not stay with her, there's no harm in learning about polyamory. Knowledge is a good thing, and familiarity may help ease some of the discomfort you're feeling.
Telling people you're poly isn't an easy thing; many poly people are "in the closet" about it. Telling someone you're dating is especially difficult, but I admit it's something that needs to be done, and a couple of months seems a bit long for her to wait before telling you. I'm just saying you might want to extend some pardon to her on this point; put yourself in her shoes, she probably had a hard time figuring out how and when to tell you.
If you get along well with the both of them, then you have a good thing that shouldn't be quickly tossed aside. Give it a chance. Learn about polyamory. Explore this site and find out what specific questions you may have. Post as needed, and people will respond.
I just think you've had a bit of a shock, and need some time to think about it. It's okay to invest a little more time in this, most things in life that are worthwhile involve a certain amount of investment/risk.
Give all the above posts in this thread some thought; they're good posts and have good advice.
Love means never having to say, "Put down that meat cleaver!"