It's quite easy. While you were in the relationship, your needs weren't fulfilled. You kind of shut yourself from the possibility of leaving the relationship. Therefore your brain, while looking for a way to get your needs met without leaving the relationship, thought about going poly.
Once you left, you realized that you don't need to be poly. A healthy monogamous partner is enough. You were shut off from that possibility as long as you were in that relationship. Once you left, this has become an option.
Congratulations and have fun with your new-found sexual freedom
Ralph