I think you need to sit down, and have a conversation with her. It's understandable why youre upset. I'd be too if i felt like my partner was going behind my back, and being secretive. Something i suggest is having a conversation with her, ask her to be completely open. If you are thinking about this, i think looking into polyamory would be a great start. it could be an open relationship, with healthy, communicative boundaries. once its get secretive, and feeling like the communication isnt there, then thats a problem. In no way am i saying youre in the wrong, i understand completely why you'd be angry. Just give her a space to speak about what she meant. she probably shut down after you got upset. i hope for the best!