If she's not engaging in relationships with her sex partners, I wouldn't call it polyamory. I'd classify that as swinging, myself. If it's just about the sex, it isn't poly.
As for mono/poly matchups, yes there are people who have those. One partner is satisfied with the single relationship and the other prefers to have multiple. The terms of those relationships are negotiated on an individual basis in a manner that addresses all of the issues of the couple involved.
So, I reckon you need to address the issue with her. From my point of view, her having a one night stand once a month doesn't qualify as frequent; for you, it obviously does. I'll offer up some questions for you to ponder before discussing the issue with her. (And I'll offer that quick, flippant answers to the questions won't help a bit. Some serious, prolonged thought is called for.)
What does it mean to you for her to have sex with somebody else once a month?
What would happen if she continues to do that?
What wouldn't happen if she continues to do that?
What would happen if she doesn't do that?
What wouldn't happen if she doesn't do that?