It's not about you. She met someone and felt something for him. Didn't mean she loves you any less. Love doesn't have a limited amount for a person to give.
And you are right...she did tell you that open relationships were her thing from the get-go. That's a big piece of info you shouldn't have ignored.
As for jealousy, well, another way to spell jealousy is F E A R. You've got to learn to trust her that when she says she loves you, and that you're her primary partner, she means it.