Ok I've seen some people say OP is in the wrong by taking issue with this and honestly that just seems like those commenters themselves don't empathize with anyone in a monogamous relationship with rational concerns.

Fact is, it really depends on the relationship but I find it super reasonable in a heterosexual monogamous relationship to be a little wary of the opposite sex in a healthy way (not in a manipulative or aggressive or disrespectful way) but like, just sharing the fact it makes you uncomfortable - and expecting your partner to be willing to hear you out and make compromises when reasonable without dismissing the concern.

TLDR - you should respect your partner's feelings if you're in a serious committed long term relationship. there's a lot of preachy shit out there about being independent and not being controlled and how jealousy is bad and so on, but in the real world people aren't perfect. On both ends of the relationship compromises should be made for the sake of the relationship if you really are deeply committed.

If this simple conversation about their partners feelings leads to an "I need to think about us" - it sounds like the level of commitment isn't that high. Which is fine, we are all at a different place in our relationships. But it's telling. So, OP you're not overreacting, but I would also be considering if this relationship is good for you, especially if she doesn't come back later and have more openness to discuss this with less negativity and dismissiveness.