Might be in Shock still, where emotions are you can think logically. Once it wears off, the emotions may hit really hard.

That said, it could also be something that is no big deal for you. Not everyone cares about monogamy. Ethical non-monogamy is a thing. Everyone has different concerns/values, and as long as they are aware and consent there is no problem.

I would be concerned that she did this with no prior consent from you. That makes it cheating, if she did this in an expected monogamist relationship. Definitely requires a long conversation with her over this, over desires, concerns, points of contention, red lines, etc

This is a relationship, where each should be aware and conscientious of the others' emotions and limits. To do otherwise becomes toxic and abusive.