I'm 99% sure this was just an oversight. I've had parties where I thought I invited everyone but then, when they weren't there, I called them and said, "Hey, man, where WERE you?" only to find out they had no idea there even was party.
Guest lists are made up in our heads. You were having relationship problems and never engaged regarding the wedding. She might have thought you were hoping NOT to have to go. Therefore, when the wedding guest list was made, her head made no connection to you.
Oversight. (Or she thinks you're the on that keeps eating her lunch out of the break room refrigerator.)Buy her a wedding gift. Give it to her, congratulate her, and tell her it's ok that you didn't get invited. She will either explain or you will be the better person of the situation, either way, you get closure.