My FWB is mad because I’ve started to hookup with her friend after we had a threesome

So yeah, basically what the title says. My FWB got her friend involved in our bedroom life and we had a really good time and me and the friend exchanged numbers and we’ve hooked up a few times ourselves. But now my fwb is mad about it and says I’m betraying her. We’re not in a relationship though and she was the one who actually invited her friend into the bed anyway. I just don’t know what I’m doing wrong here