I'm sitting here wondering if she's polyamorous.
Stop wondering, that is a hell of an assumption to even consider, and if you keep that thought active in your head, you risk making a decision that not only blows up in your face but also creating a huge mess that affects many around you.
Correct.
Look, it is simple, her current personal relationship is none of your business, and by speculating and allowing yourself to assign the label "polyamorous" to her, you involve yourself in her relationship, you also start building a mental version of her that is detached from reality if you let it go far enough.
This will cause you to act irrationally and increase the heartache once the inevitable blow up happens.