You need to pick your lane and stay in it. Do you want exclusivity, or do you want an open relationship?I honestly wouldn't care if she hooks up with other people, and I'm not against hooking up with other people. I was seeing two other people when we first started dating but those naturally gave way to my interest in her but I'm not opposed to hooking up with others casually either. My problem is with the sudden change in behavior and distance I'm feeling that wasn't there before. But I do think it would be worth hashing out because maybe we weren't on the same page about what seeing other people meant. I am happy with a serious but open relationship and it's something I've thought about a lot recently. We were so heavily invested in each other's day and weekends though that I find it hard to believe she was pursuing anything more serious with anyone else until possibly recently. But I do feel like once someone starts fading I rarely ever see it recover so maybe this is all moot.
In my experience, whenever "asking me .... we are still on the same page about seeing other people" happens, that's a Define the Relationship moment. Given her reaction, I don't think you responded the way she wanted you to...